Metal valves can be said to be the key component structures that are most prone to corrosion and failure in engineering equipment. Usually, the sealing surface, valve stem, diaphragm, small spring and other valve parts of metal valves are generally made of first-class materials, and the valve body and valve cover are suitable for For secondary or tertiary materials, for valves used in high pressure, highly toxic, flammable, explosive, and radioactive media, use less corrosive materials.
Under complex working conditions such as atmosphere or solution, metal valves not only have uniform corrosion on the metal surface at all times, but also pitting corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue at local locations of the metal. Corrosion, selective corrosion, wear corrosion, cavitation corrosion, friction corrosion, hydrogen corrosion and other localized corrosion.
Anti-corrosion measures for metal valves
1 Choose corrosion-resistant materials according to the corrosive medium
In the actual production, the corrosion of the medium is very complicated. 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 different. The medium temperature is increased by 10 ℃ every time. , the corrosion rate increases by about 1 to 3 times. The medium concentration has a great influence on the corrosion of valve materials. For example, when lead is in sulfuric acid with a small concentration, the corrosion is very small. When the concentration exceeds 96%, the corrosion increases sharply. On the contrary, carbon steel, When the concentration of sulfuric acid is about 50%, the corrosion is the most serious. When the concentration increases to more than 6%, the corrosion decreases sharply. For example, aluminum is very corrosive in concentrated nitric acid with a concentration of more than 80%, but in medium and low concentrations of nitric acid. The corrosion is serious in the middle. 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 examples that the correct selection of valve materials should be based on specific conditions, analyze various factors affecting corrosion, and select materials according to relevant anti-corrosion manuals.
2 using 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, bonnet, lining, sealing surface, etc. are commonly made of non-metallic materials , As for gaskets, the fillers are mainly made of non-metallic materials. Plastics such as PTFE and chlorinated polyether are used as valve linings, and rubbers such as natural rubber, neoprene, and nitrile rubber are used as valve linings. The main body of the bonnet is 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 performance of rubber. Now more and more Yes, nylon, polytetrafluoroethylene and other plastics are used, and natural rubber and synthetic rubber are used to make various sealing surfaces and sealing rings, which are used on various types of valves. These non-metallic materials used as sealing surfaces are not only resistant to Good corrosion and good sealing performance, especially suitable for use in medium with particles. Of course, their strength and heat resistance are low, and the scope of application is limited. The emergence of flexible graphite has made non-metals enter the high temperature field, It solves the problem of packing and gasket leakage that is difficult to solve for a long time, and it is a good high temperature lubricant.
3 metal surface treatment
Valve connecting screws are commonly treated with galvanized, chrome-plated, and oxidized (blued) to improve the ability to resist atmospheric and medium corrosion. In addition to the above-mentioned methods for other fasteners, surface treatments such as phosphating are also used according to the situation.
The sealing surface and the closing parts with small caliber often use surface technology such as nitriding and boronizing to improve its corrosion resistance and wear resistance. The valve disc made of 38CrMoAlA, the nitriding layer is ≥0.4mm.
The valve stem anti-corrosion widely adopts surface treatment processes such as nitriding, boronizing, chrome plating and nickel plating to improve its corrosion resistance, corrosion resistance and abrasion resistance. Different surface treatments should be suitable for different valve stem materials and In the working environment, the valve stem in contact with the asbestos filler in the atmosphere, water vapor medium, can use hard chromium plating, gas nitriding process (stainless steel is not suitable for ion nitriding process); the valve in the hydrogen sulfide atmospheric environment uses electroplating high phosphorus nickel The coating has good protection performance; 38CrMoAlA can also resist corrosion by ion and gas nitriding, but it is not suitable to use hard chromium coating; 2Cr13 can resist ammonia corrosion after quenching and tempering, and carbon steel using gas nitriding can also resist ammonia corrosion, And all phosphorus-nickel coatings are not resistant to ammonia corrosion; the 38CrMoAlA material after gas nitridation has excellent corrosion resistance and comprehensive performance, and it is used to make valve stems.
Small-diameter valve bodies and handwheels are often chrome-plated to improve their corrosion resistance and decorate the valve.
4 Thermal spray
Thermal spraying is a type of process block for preparing coatings, and it has become one of the new technologies for material surface protection. It uses high energy density heat sources (gas combustion flame, electric arc, plasma arc, electric heat, gas explosion, etc.) After the non-metallic material is heated and melted, it is sprayed onto the pretreated basic surface in the form of atomization to form a spray coating, or at the same time, the basic surface is heated so that the coating is melted again on the surface of the substrate to form a 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 on metallic or non-metallic substrates by one or more thermal spraying methods.
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 coatings, with heat insulation, insulation (or different electricity), wearable sealing, self-lubricating, heat radiation, Special properties such as electromagnetic shielding; parts can be repaired by thermal spraying.
5 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. Coating is also a non-metallic material, which is usually prepared from synthetic resin, rubber slurry, vegetable oil, solvent, etc. , cover the metal surface, isolate the medium and the atmosphere, and achieve the purpose of anti-corrosion. The coating is mainly used in environments with less corrosion 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. Corrosion to the valve. The paint is mixed with different colors to represent the materials used by Faen. The valve is sprayed with paint, generally once every six months to one year.
6 Add corrosion inhibitor
The mechanism of corrosion control by corrosion inhibitor is that it promotes the polarization of the battery. The corrosion inhibitor is mainly used in the medium and filler. Adding corrosion inhibitor to the medium can slow down the corrosion of equipment and valves. In oxygen-containing sulfuric acid, it is in a cremated state within a large solubility range, and the corrosion is more serious, but adding a small amount of oxidants such as copper sulfate or nitric acid can make the stainless steel passivated, and a protective film will be formed on the surface to prevent the immersion of the medium. In hydrochloric acid, if a small amount of oxidant is added, the corrosion of titanium can be reduced. Water is often used as the pressure test medium for valve pressure testing, which is easy to cause valve corrosion. Adding a small amount of sodium nitrite in water can prevent water from corroding the valve. The asbestos packing contains chlorides, which corrode the valve stem greatly. If the method of washing with distilled water is used, the content of chlorides can be reduced, but this method is very difficult to implement and cannot be generally promoted. The ester is suitable for special needs.
In order to protect the valve stem and prevent the corrosion of the asbestos packing, in the asbestos packing, the valve stem is filled with corrosion inhibitor and sacrificial metal. The corrosion inhibitor is composed of sodium nitrite and sodium chromate, which can make the surface of the valve stem form a passivation layer film to improve the corrosion resistance of the valve stem; the solvent can slowly dissolve the corrosion inhibitor, and it can play a lubricating role; adding zinc powder to asbestos as a sacrificial metal, in fact, zinc is also a corrosion inhibitor, it can First of all, it is combined with the chloride in asbestos, so that the chance of contact between the chloride and the valve stem metal is greatly reduced, so as to achieve the purpose of anti-corrosion. atmospheric corrosion.
7 Electrochemical protection
There are two types of electrochemical protection: anodic protection and cathodic protection. If zinc is used to protect iron, zinc is corroded, and zinc is called a sacrificial metal. In production practice, anodic protection is used less, and cathodic protection is used more. Large valves and important valves Using this cathodic protection method is an economical, simple and effective method. Zinc is added to asbestos filler to protect the valve stem, which also belongs to the cathodic protection method.
8 Control the corrosive environment
The so-called environment has two broad senses and narrow senses. The broad environment refers to the environment around the valve installation site and its internal circulation medium; the narrow sense environment refers to the conditions around the valve installation site. Most environments cannot be controlled, and the production process cannot be arbitrary. Changes. Only in the case of no damage to products and processes, environmental control methods can be used, such as deoxidizing boiler water, adjusting PH value of domestic alkali in oil refining process, etc. From this point of view, the above-mentioned addition of corrosion inhibition Agents, electrochemical protection, etc. are also controlled corrosive environments.
The atmosphere is full of dust, water vapor and smoke, especially in the production environment, such as smoke halogen, toxic gases and fine powder emitted by the equipment, which will corrode the valve to varying degrees. Cleaning, purging the valve, and refueling regularly are effective measures to control environmental corrosion. The valve stem is installed with a protective cover, the ground valve is set up well, and the valve surface is sprayed with paint. These are all methods to prevent corrosive substances from eroding the valve. Ambient temperature Elevation 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 used as much 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 is considered from the design. A valve product with a reasonable structure design and correct process method. Undoubtedly, it has a good effect on slowing down the corrosion of the valve.
Therefore, the design and manufacturing department should improve the parts that are unreasonable in structural design, incorrect in process method, and easy to cause corrosion, so as to make them suitable for the requirements of various working conditions. For different types of valve parts corrosion, small We have a trick:
The methods of preventing intergranular corrosion of austenitic stainless steel valve parts include: "solution quenching" treatment, that is, heating to about 1100 ° C for water quenching, selecting austenitic stainless steel containing titanium and niobium, and carbon content below 0.03% , reduce the production of chromium carbide.
Stress corrosion occurs when corrosion and tensile stress occur at the same time. The method 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, Spray anti-corrosion paint. Add corrosion inhibitor, apply compressive stress and other measures.
Abrasion corrosion is a form of corrosion caused by the alternating action of fluid on metal wear and corrosion. It is a common corrosion of valves. This corrosion occurs mostly on the sealing surface. Prevention methods: choose corrosion-resistant and wear-resistant materials , Improve the structural design, adopt cathodic protection, etc.
Frictional corrosion is that the two parts in contact with each other bear loads at the same time, and the contact surface is damaged due to vibration and sliding. Frictional corrosion occurs at the bolt connection, the connection between the valve stem and the closing part, between the ball bearing and the shaft. It can be protected by applying lubricating grease, reducing friction, surface phosphating, selecting cemented carbide, and improving surface hardness by spraying tiles or cold working.
After welding, try to take corresponding protective measures such as annealing treatment. Improve the surface roughness of the valve stem and the surface roughness of other valve parts. The higher the surface roughness level, the stronger the corrosion resistance. Improve the processing technology and structure of packing and gasket , The use of flexible graphite and plastic packing, as well as flexible graphite adhesive gaskets and PTFE-coated gaskets can improve sealing performance and reduce the corrosion of the valve stem and flange sealing surface