Metal valves can be said to be key components and structures that are prone to corrosion and failure in engineering equipment. Generally, the sealing surface, valve stem, diaphragm, small spring, and other valve components of metal valves are made of materials, while the valve body, valve cover, and other materials are suitable for two or three materials. For valves used in high-pressure, highly toxic, flammable, explosive, and radioactive media, materials with very low corrosion resistance are selected.
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 leaching, abrasion corrosion, cavitation corrosion, friction corrosion, hydrogen corrosion and other local corrosion at local locations of the metal.
Corrosion prevention measures for metal valves
01. Select corrosion-resistant materials based on corrosive media
In practical production, the corrosion of the medium is very complex. Even if the valve material used in the medium is different in concentration, temperature, and pressure, the corrosion of the medium to the material is also different. For every 10 ℃ increase in medium temperature, the corrosion rate increases by approximately 1-3 times. The concentration of the medium has a significant impact on the corrosion of valve materials. For example, when lead is in sulfuric acid with low concentration, the corrosion is very small. When the concentration exceeds 96%, the corrosion increases sharply. On the contrary, carbon steel exhibits severe corrosion when the sulfuric acid concentration is around 50%, and when the concentration increases to over 6%, the corrosion actually sharply decreases. Aluminum is highly corrosive in concentrated nitric acid with a concentration of over 80%, but it corrodes severely in medium to low concentrations of nitric acid. Although stainless steel has strong corrosion resistance to dilute nitric acid, its corrosion becomes more severe in concentrated nitric acid of over 95%.
From the above examples, it can be seen that the correct selection of valve materials should be based on specific situations, analyzing various factors that affect corrosion, and selecting materials according to relevant anti-corrosion manuals.
02. Using non-metallic materials
Non metal has good corrosion resistance. 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, cover, lining, sealing surface, and other commonly used non-metallic materials are used for making valves. As for gaskets, fillers are 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 rubber, while the body and bonnet body are made of ordinary cast iron and carbon steel. This not only ensures the strength of the valve, but also ensures that the valve is not corroded. The pinch valve is also designed based on the excellent corrosion resistance and abnormal performance of rubber. Nowadays, it is becoming increasingly popular to use plastics such as nylon and polytetrafluoroethylene, as well as natural rubber and synthetic rubber to make various sealing surfaces and rings for various valves. These non-metallic materials used as sealing surfaces not only have good corrosion resistance but also 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 emergence of flexible graphite has brought non-metals into the high-temperature domain, solving the long-standing problem of filler and gasket leakage, and is a good high-temperature lubricant.
03. Metal surface treatment
Valve connecting screws are commonly treated with zinc plating, chromium plating, and oxidation (blueing) to improve their resistance to atmospheric and medium corrosion. In addition to using the above methods for treatment, other fasteners also undergo surface treatment such as phosphating according to the situation.
Sealing surfaces and small diameter closure components often use surface processes such as nitriding and boronizing to improve their corrosion resistance and wear resistance. The valve disc made of 38CrMoAlA has a nitrided layer of ≥ 0. 4mm.
The anti-corrosion of valve stems widely adopts surface treatment processes such as nitriding, boronizing, chrome plating, and nickel plating to improve their corrosion resistance, corrosion resistance, and abrasion resistance. Different surface treatments should be suitable for different valve stem materials and working environments. For valve stems that come into contact with asbestos fillers in atmospheric and water vapor media, hard chromium plating and gas nitriding processes can be used (ion nitriding processes are not suitable for stainless steel); The use of electroplating high phosphorus nickel coating for valves in hydrogen sulfide atmosphere has good protective performance; 38CrMoAlA can also resist corrosion by using ion and gas nitriding, but hard chromium coating is not suitable; 2Cr13 can withstand ammonia corrosion after quenching and tempering, and carbon steel using gas nitriding can also withstand 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 commonly used to make valve stems.
Small diameter valve bodies and handwheels 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 coatings, which has become one of the new technologies for surface protection of materials. It is a surface strengthening process that utilizes high energy density heat sources (such as gas combustion flames, arcs, plasma arcs, electric heating, gas explosion, etc.) to heat and melt metal or non-metallic materials, and then spray them in the form of atomization onto the pre treated basic surface to form a spray coating, or simultaneously heat the basic surface to melt the coating again on the substrate surface, forming 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 more 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-lubricating, thermal radiation, electromagnetic shielding, etc; Parts can be repaired by Thermal spraying.
05. Spray coating
Coating is a widely used anti-corrosion method, and it is an indispensable anti-corrosion material and identification mark on valve products. Coatings are also non-metallic materials, usually formulated from synthetic resins, rubber slurries, vegetable oils, solvents, etc., which cover the metal surface, isolate the medium and atmosphere, and achieve anti-corrosion purposes. Coatings are mainly used in environments with less severe corrosion such as water, salt water, seawater, and atmosphere. The inner cavity of the valve is commonly coated with anti-corrosion 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, usually every six months to a year.
06. Adding corrosion inhibitors
The mechanism by which corrosion inhibitors control corrosion is that they promote the formation of batteries. Corrosion inhibitors are mainly used in media and fillers. Adding corrosion inhibitor in the medium can slow down the corrosion of equipment and valves. For example, chromium nickel stainless steel is cremated within 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(II) sulfate or nitric acid can make the stainless steel turn passive, and the surface forms a protective film to prevent the corrosion of the medium. If a small amount of oxidant is added in hydrochloric acid, the corrosion of titanium can be reduced. The valve pressure test usually uses water as the medium for pressure test, which is easy to cause corrosion of the valve. Adding a small amount of Sodium nitrite in the water can prevent the corrosion of water on the valve. The asbestos packing contains chlorides, which cause significant corrosion to the valve stem. If distilled water washing method is used, the content of chlorides can be reduced. However, this method is difficult to implement and cannot be widely promoted. Esters are suitable for special needs.
To protect the valve stem and prevent corrosion of asbestos packing, a corrosion inhibitor and sacrificial metal are applied to the valve stem 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 stem to improve the corrosion resistance of the valve stem; Solvents can slowly dissolve corrosion inhibitors and provide lubrication; Adding zinc powder as sacrificial metal to asbestos, in fact, zinc is also a corrosion inhibitor. It can first combine with the chlorides in asbestos, greatly reducing the chance of chlorides coming into contact with the valve stem metal, thus achieving anti-corrosion purposes. If corrosion inhibitors such as red lead and calcium lead are added to the coating, spraying on the surface of the valve can prevent atmospheric corrosion.
07. Electrochemical protection
There are two types of electrochemical protection: positive protection and negative protection. If zinc is used to protect iron, it corrodes and is called a sacrificial metal. In production practice, positive protection is less commonly used, while negative protection is more commonly used. The use of this cathodic protection method for large and important valves is an economical, simple, and effective method. Adding zinc to asbestos packing to protect the valve stem also belongs to the cathodic protection method.
08. Control of corrosive environment
The so-called environment can be broadly or narrowly defined, which refers to the environment around the valve installation site and its internal circulation medium; Narrowly defined environment refers to the conditions around the valve installation site. Most environments are uncontrollable, and production processes cannot be arbitrarily changed. Only when there is no harm to the product, process, etc., can environmental control methods be used, such as deoxidation of boiler water, alkali adjustment of pH value in refining processes, etc. From this perspective, the addition of corrosion inhibitors and electrochemical protection mentioned above also belongs to the controlled corrosion environment.
The atmosphere is filled with dust, water vapor, and smoke, especially in production environments, such as smoke halides, toxic gases and micro powders emitted by equipment, which can cause varying degrees of corrosion to valves. Operators should regularly clean, blow out valves, and refuel according to the operating procedures, which is an effective measure to control environmental corrosion. The installation of a protective cover for the valve stem, the installation of a well for the ground valve, and the painting of the valve surface are all methods to prevent corrosive substances from corroding the valve. Elevated environmental temperature and air pollution, especially for equipment and valves in enclosed environments, can accelerate their corrosion. Open factory buildings or ventilation and cooling measures should be adopted as much as possible to slow down environmental corrosion.
09. Improve processing technology and valve structure form
The anti-corrosion protection of valves is a problem that is considered from the beginning of design, and a valve product with a reasonable structural design and correct process methods. Undoubtedly, it has a good effect on slowing down the corrosion of valves.
Therefore, the design and manufacturing departments should improve the components that have unreasonable structural design, incorrect process methods, and are prone to corrosion to meet the requirements of various working conditions.
10. For different types of corrosion in valve components, the editor has suggestions:
The method to prevent intergranular corrosion of stainless steel valve components is to undergo "solid solution quenching" treatment, which involves heating to around 1100 ℃ and water quenching. Austenitic stainless steel containing titanium and niobium, with a carbon content below 0.03%, is selected to reduce the generation of chromium carbide.
Stress corrosion occurs under the simultaneous action of corrosion and tensile stress, and the method to prevent stress corrosion is to eliminate or reduce the stress generated during welding and cold processing through heat treatment, improve unreasonable valve structures, avoid stress concentration, and use electrochemical protection and spray anti-corrosion coatings. Measures such as adding corrosion inhibitors and applying compressive stress.
Wear corrosion is a type of corrosion formed by the alternating action of fluid on metal wear and corrosion, and is a common type of corrosion in valves. This type of corrosion mainly occurs on the sealing surface. Prevention method: Select corrosion-resistant and wear-resistant materials, improve structural design, and use cathodic protection.
Frictional corrosion refers to the damage caused by vibration and sliding on the contact surface of two components that are in contact with each other and bear loads simultaneously. Friction corrosion occurs at bolted connections, valve stem to closure connections, and between ball bearings and shafts. Protection can be achieved by applying lubricating grease, reducing friction, surface phosphating, selecting hard alloys, and using spray coating or cold working to improve surface hardness.
After welding, corresponding protective measures such as annealing treatment should be adopted as much as possible to improve the surface roughness of the valve stem processing and other valve components. The higher the surface roughness, the stronger the corrosion resistance. Improving the processing technology and structure of fillers and gaskets, using flexible graphite and plastic fillers, as well as using flexible graphite adhesive gaskets and polytetrafluoroethylene wrapped gaskets, can improve sealing performance and reduce corrosion on the sealing surfaces of the valve stem and flange.
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