Do you know the ten best ways to prevent corrosion of metal valves?

Metal valve can be said to be the key component structure that is prone to corrosion failure in engineering equipment. Generally, the sealing surface, valve rod, diaphragm, small spring and other valve parts of metal valve are generally made of Class I materials, the valve body, valve cover and other materials are suitable for Class II or III materials, and the valves used for high pressure, highly toxic, flammable, explosive and radioactive media are made of materials with low corrosivity.
Under complex working conditions such as atmosphere or solution, metal valves are not only subject to uniform corrosion on the metal surface at all times, but also prone to pitting corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, wear corrosion, cavitation corrosion, friction corrosion, hydrogen corrosion and other local corrosion at local locations of the metal.
01 Select corrosion-resistant materials according to corrosive media
In production practice, the corrosion of medium is very complex. Even if the valve material used in the medium is different in concentration, temperature and pressure, the corrosion of medium to material is also different. The corrosion rate will increase by 1~3 times when the medium temperature increases by 10 ℃. The medium concentration has a great impact 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%. When the concentration increases to more than 6%, the corrosion decreases sharply. For example, aluminum is highly corrosive in concentrated nitric acid with a concentration of more than 80%, but it is severely corrosive in medium and low concentrations of nitric acid. Although stainless steel has strong corrosion resistance to dilute nitric acid, its corrosion is aggravated 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, analysis of various factors affecting corrosion, and selection of materials according to the relevant anti-corrosion manual.
02 Use of non-metallic materials
Non-metallic corrosion resistance is good. 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 filler is mainly made of non-metallic materials. The valve lining is made of polytetrafluoroethylene, chlorinated polyether and other plastics, as well as natural rubber, neoprene, nitrile rubber and other rubbers, while the valve body and bonnet body are made of ordinary cast iron and carbon steel. This ensures the strength of the valve and the corrosion resistance of the valve. The pinch valve is also designed according to the good corrosion resistance and abnormal performance of rubber. Now 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 for various valves. These non-metallic materials used as sealing surfaces not only have good corrosion resistance, but also have good sealing performance, especially suitable for use in granular media. Of course, their strength and heat resistance are low, and their application range is limited. The appearance of flexible graphite makes non-metals enter the high temperature region, solves the problem of packing and gasket leakage which is difficult to solve for a long time, and is a good high-temperature lubricant.
03 Metal surface treatment
Valve connecting bolts are usually galvanized, chrome-plated and oxidized (bluing) to improve the ability of atmospheric and medium corrosion resistance. In addition to the above methods, other fasteners also adopt surface treatment such as phosphating according to the situation.
The sealing surface and closing parts with small diameter often adopt surface processes such as nitriding and boronizing to improve their corrosion resistance and wear resistance. The valve disc made of 38CrMoAlA has nitrided layer ≥ 0. 4mm.
The surface treatment processes such as nitriding, boronizing, chrome plating and nickel plating are widely used to improve the corrosion resistance, corrosion resistance and abrasion resistance of the valve rod. Different surface treatments shall be suitable for different valve rod materials and working environments. Hard chromium plating and gas nitriding process can be used for the valve rod contacting with asbestos packing in the atmosphere and water vapor medium (ion nitriding process is not suitable for stainless steel); The valve in hydrogen sulfide atmosphere has better protective performance by adopting electroplating high phosphor nickel coating; 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, and carbon steel with gas nitriding can also resist ammonia corrosion, while all phosphor-nickel coatings are not resistant to ammonia corrosion; After gas nitriding, 38CrMoAlA material has good corrosion resistance and comprehensive performance. It is widely used to make valve stems.
The valve body and handwheel with small diameter are often chrome plated to improve their corrosion resistance and decorate the valve.
04 Thermal spraying
Thermal spraying is a kind of process block for preparing coating, and has become one of the new technologies for material surface protection. It is a surface strengthening process method that uses high energy density heat source (gas combustion flame, arc, plasma arc, electric heating, gas explosion, etc.) to heat and melt metal or non-metallic materials, and then spray them to the pretreated basic surface in the form of atomization to form a spraying layer, or at the same time heat the basic surface to melt the coating on the base surface again to form a spray welding layer. Most metals and their alloys, metal oxide ceramics, metal ceramic composites and hard metal compounds can be coated on metal or non-metallic substrates by one or several thermal spraying methods.
Thermal spraying can improve its surface corrosion resistance, wear resistance, high temperature resistance and other properties, and extend its service life. Thermal spraying special functional coating with special properties such as heat insulation, edge (or abnormal electricity), grindable sealing, self-lubrication, thermal radiation, electromagnetic shielding, etc; Parts can be repaired by thermal spraying.
05 Painting
Paint is a widely used anti-corrosion means, and is an indispensable anti-corrosion material and identification mark on valve products. Coatings are also non-metallic materials, which are usually prepared by synthetic resin, rubber slurry, vegetable oil, solvent, etc., and are covered on the metal surface to isolate the medium and atmosphere to achieve the purpose of corrosion prevention. The coating is mainly used in water, salt water, sea water, atmosphere and other environments with low corrosion. The inner cavity of the valve is usually coated 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 Farn. The valve is sprayed with paint every six months to every year.
06 Add corrosion inhibitor
The mechanism of corrosion inhibitor controlling corrosion is that it promotes the battery. The corrosion inhibitor is mainly used at the medium and filler. Adding corrosion inhibitor in the medium can slow down the corrosion of equipment and valves. For example, chromium-nickel stainless steel becomes cremated in a large solubility range in oxygen-free sulfuric acid, and the corrosion is relatively serious. However, adding a small amount of oxidants such as copper sulfate or nitric acid can make the stainless steel change into a passive state, and the surface forms a protective film to prevent the corrosion of the medium. In hydrochloric acid, adding a small amount of oxidant can reduce the corrosion of titanium. Water is often used as the medium for pressure test of valves, which is easy to cause corrosion of valves. Adding a small amount of sodium nitrite in water can prevent water corrosion of valves. The asbestos packing contains chlorides, which cause great corrosion to the valve rod. If the method of distilled water washing is used, the content of chlorides can be reduced. However, this method is difficult to implement and cannot be popularized generally. The ester is suitable for special needs.
In order to protect the valve rod and prevent the corrosion of asbestos packing, the valve rod is coated with corrosion inhibitor and sacrificial metal in the asbestos packing. The corrosion inhibitor consists of sodium nitrite and sodium chromate, which can form a passivation film on the surface of the valve rod and improve the corrosion resistance of the valve rod; The solvent can slowly dissolve the corrosion inhibitor and play a lubricating role; Zinc powder is added to asbestos as 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 contact between the chloride and the valve rod metal, thus achieving the purpose of corrosion prevention. If corrosion inhibitors such as red lead and calcium lead are added to the coating, spraying on the valve surface can prevent atmospheric corrosion.
07 Electrochemical protection
There are two kinds of electrochemical protection: positive protection and negative protection. If iron is protected by zinc, zinc is corroded, and zinc is called sacrificial metal. In production practice, the application of positive protection is less than that of negative protection. This cathodic protection method is an economical, simple and effective method for large valves and important valves. Zinc is added to asbestos packing to protect valve rod, which also belongs to cathodic protection method.
08 Control of corrosive environment
The so-called environment has two broad and narrow senses. The broad sense refers to the environment around the valve installation and its internal circulation medium; In a narrow sense, the environment refers to the conditions around the valve installation site. Most environments cannot be controlled, and the production process cannot be changed at will. Environmental control methods, such as boiler water deoxidation and alkali adjustment of PH value in oil refining process, can be used only if there is no damage to products and processes. From this point of view, the above addition of corrosion inhibitor and electrochemical protection also belong to the controlled corrosion environment.
The atmosphere is full of dust, water vapor and smoke, especially in the production environment, such as halogen, toxic gases and micro-powder emitted by equipment, which will cause different degrees of corrosion to the valve. The operators shall regularly clean and purge the valves and refuel regularly according to the regulations in the operating procedures, which is an effective measure to control environmental corrosion. The valve rod is installed with a protective cover, the ground valve is installed with a ground well, and the valve surface is sprayed with paint, which are all methods to prevent corrosive substances from corroding the valve. Environmental temperature rise and air pollution, especially for equipment and valves in closed environment, will accelerate their corrosion. Open workshop or ventilation and cooling measures shall be adopted as far as possible to reduce environmental corrosion.
09 Improve processing technology and valve structure
Anti-corrosion protection of valves is a problem that should be 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 valves.
Therefore, the design and manufacturing departments should improve the parts that are unreasonable in structure design, incorrect in process method and easy to cause corrosion to meet the requirements of various working conditions.
10 Corrosion types for different valve components
The method to prevent intergranular corrosion of stainless steel valve parts is to carry out "solution quenching" treatment, that is, heating to about 1100 ℃ for water quenching, selecting austenitic stainless steel containing titanium and niobium and carbon content below 0.03% to reduce the generation of chromium carbide.
Stress corrosion occurs under the simultaneous action of corrosion and tensile stress. The method to prevent stress corrosion is to eliminate or reduce the stress generated during welding and cold processing through heat treatment, improve the unreasonable valve structure, avoid stress concentration, and use electrochemical protection and spray anti-corrosion coating. Add corrosion inhibitor, apply compressive stress and other measures.
Abrasion corrosion is a kind of corrosion form produced by the alternating action of fluid wear and corrosion on metal. It is a common kind of corrosion of valves. This kind of corrosion mainly occurs on the sealing surface. Prevention methods: select corrosion-resistant and wear-resistant materials, improve structural design, and adopt cathodic protection.
Frictional corrosion refers to the damage of the contact surface caused by vibration and sliding when two parts in contact bear the load at the same time. Frictional corrosion occurs at the bolted connection, the connection between the valve rod and the closure, and between the ball bearing and the shaft. It can be protected by applying lubricating grease, reducing friction, phosphating the surface, selecting cemented carbide, and improving the surface hardness by tile spraying or cold processing.
After welding, annealing treatment and other corresponding protective measures shall be taken as far as possible to improve the machined surface roughness of the valve rod and the surface roughness of other valve parts. The higher the surface roughness, the stronger the corrosion resistance. Improving the processing technology and structure of packing and gasket, using flexible graphite and plastic packing, as well as flexible graphite adhesive gasket and polytetrafluoroethylene wrapped gasket, can improve the sealing performance and reduce the corrosion of the sealing surface of the ground valve rod and flange.