Metal valves can be said to be the most prone to corrosion and failure of the key component structure in engineering equipment. Usually, the sealing surface, valve stem, diaphragm, small spring and other valve parts of metal valves are generally made of first-grade materials, and the valve body and bonnet are suitable for Secondary or tertiary materials, for valves with high pressure, highly toxic, flammable, explosive, and radioactive media, use less corrosive materials.
valve corrosion
Under complex working conditions such as atmosphere or solution, metal valves not only undergo uniform corrosion on the metal surface all the time, but are also prone to pitting corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, and fatigue in local parts of the metal. Corrosion, selective corrosion, abrasion corrosion, cavitation corrosion, friction corrosion, hydrogen corrosion and other localized corrosion.
Anti-corrosion measures for metal valves
1. Select corrosion-resistant materials according to the corrosive medium
In 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 material by the medium is different. When the temperature of the medium increases by 10°C, the corrosion rate increases by about 1~3 times. The concentration of the medium has a great influence on the corrosion of the valve material. For example, lead is in sulfuric acid with a small concentration, and the corrosion is very small. When the concentration exceeds 96%, the corrosion is rapid. Contrary to 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 drops sharply. For example, aluminum is very corrosive in concentrated nitric acid with a concentration of more than 80%, but Corrosion is serious in medium and low concentrations of nitric acid. Although stainless steel has strong corrosion resistance to dilute nitric acid, the corrosion will be 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. of the valve are usually made of non-metallic materials, and the gaskets and packing are mainly made of non-metallic materials. Plastics such as polytetrafluoroethylene, chlorinated polyether, and rubber such as natural rubber, neoprene, and nitrile rubber are used as valve linings, and the main body and valve cover are made of ordinary cast iron and carbon steel. It not only improves 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 denaturation properties of rubber. Now more and more plastics such as nylon and polytetrafluoroethylene are used, and natural rubber and synthetic rubber are used to make various sealing surfaces and sealing rings for various valves. These non-metallic sealing surfaces are used as sealing surfaces. Materials, not only have good corrosion resistance, but also 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 allows non-metallic materials to enter It solves the problem of leakage of packing and gaskets that has been difficult to solve for a long time, and is a good high-temperature lubricant.
3. Metal surface treatment
Valve connection: Valve connection screws are often treated with galvanizing, chrome plating, and oxidation (blueing) to improve the ability to resist atmospheric and medium corrosion. In addition to the above-mentioned methods, other fasteners are also treated with phosphating and other surface treatments according to the situation. .
Nitriding, boronizing and other surface processes are often used for sealing surfaces and closing parts with small diameters to improve its corrosion resistance and wear resistance. The valve disc made of 38CrMoAlA has a nitrided layer ≥ 0.4mm.
Nitriding, boronizing, chrome plating, nickel plating and other surface treatment processes are widely used for valve stem anticorrosion 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 or water vapor medium can be plated with hard chromium and gas nitriding process (stainless steel should not use the ion nitriding process); the valve in the hydrogen sulfide atmospheric environment can be electroplated with high phosphorus nickel The coating has good protective performance; 38CrMoAlA can also resist corrosion by ion and gas nitriding, but it is not suitable to use hard chromium coating; 20Cr13 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; 38CrMoAlA material has excellent corrosion resistance and comprehensive performance after gas nitriding, and it is mostly used to make valve stems.
The small diameter valve body and hand wheel are also often chrome plated to improve their corrosion resistance and decorate the valve.
4. Thermal spraying
Thermal spraying is a kind of process block for preparing coatings, and has become one of the new technologies for material surface protection. It uses high energy density heat sources (gas combustion flames, electric arcs, plasma arcs, electric heating, gas explosions, etc.) to spray metal or 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 to melt the coating on the surface of the substrate again to form a surface strengthening process of the 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 its surface corrosion resistance, wear resistance, high temperature resistance, etc., and prolong its service life. Thermal spraying special functional coatings have heat insulation, insulation (or different electricity), wearable seals, self-lubrication, 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 a kind of 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. The metal surface is isolated from the medium and the atmosphere to achieve anti-corrosion purposes. The coating is mainly used in environments where water, salt water, sea water, and the atmosphere are not too corrosive. The inner cavity of the valve is often painted with anti-corrosion 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 Fine. The valves are sprayed with paint, usually every six months to one year.
6. Add corrosion inhibitor
The mechanism of corrosion inhibitors to control corrosion is that it promotes the polarization of batteries. Corrosion inhibitors are mainly used in media and fillers. Adding corrosion inhibitors to media can slow down the corrosion of equipment and valves, such as chromium-nickel stainless steel in different In oxygen-containing sulfuric acid, it will be cremated in a large range of solubility, and the corrosion is serious, but adding a small amount of oxidizing agent such as copper sulfate or nitric acid can make the stainless steel passivate and form a protective film 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 medium for pressure testing of valves, which is easy to cause corrosion of valves. Adding a small amount of sodium nitrite in water can prevent water from corroding valves. Asbestos packing contains chloride, which corrodes the valve stem greatly. If distilled water washing method is used, the chloride content can be reduced, but this method is very difficult to implement and cannot be popularized. Ester is suitable for special needs.
In order to protect the valve stem and prevent the corrosion of asbestos packing, in the asbestos packing, the valve stem is coated with corrosion inhibitor and sacrificial metal. The corrosion inhibitor consists of sodium nitrite and sodium chromate, which can make the surface of the valve stem form a passivation layer. film, improve the corrosion resistance of the valve stem; the solvent can slowly dissolve the corrosion inhibitor, and can also 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 combines with the chloride in asbestos, so that the chance of contact between the chloride and the metal of the valve stem is greatly reduced, so as to achieve the purpose of anticorrosion. If red lead, calcium lead acid and other corrosion inhibitors are added to the paint, spraying on the surface of the valve can prevent 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, less anode protection is used and more cathodic protection is used. Large valves and important valves Using this cathodic protection method is an economical, simple and effective method. Adding zinc to the asbestos filler to protect the valve stem is also a cathodic protection method.
8 Control the corrosive environment
The so-called environment has two kinds of broad sense and narrow sense. The broad sense environment refers to the environment around the valve installation and its internal circulation medium; the narrow sense environment refers to the conditions around the valve installation. Most environments cannot be controlled, and the production process cannot be arbitrary. Changes. Only in the case of no damage to the product, process, etc., methods of controlling the environment can be used, such as deoxidizing boiler water, adjusting the PH value of home alkali in the refining process, etc. From this point of view, the above-mentioned addition of corrosion inhibitors Agents, electrochemical protection, etc. also belong to the control of corrosion environment.
The atmosphere is full of dust, water vapor and smog, especially in the production environment, such as smoke bittern, toxic gas and fine powder emitted by equipment, which will corrode the valve to varying degrees. Operators should follow the regulations in the operating procedures and regularly Cleaning, purging the valve, and refueling regularly are effective measures to control environmental corrosion. Installing a protective cover on the valve stem, installing a well on the ground valve, spraying paint on the surface of the valve, etc., are all methods to prevent corrosion of the valve by corrosive substances. Ambient temperature Elevation and air pollution, especially for equipment and valves in a closed environment, will accelerate their corrosion. Open workshops should be used as much as possible or ventilation and cooling measures should be adopted to slow down environmental corrosion.
9. Improve the processing technology and valve structure
The anti-corrosion protection of valves is a problem to be considered from the beginning of design. A valve product with reasonable structural design and correct process method will undoubtedly have a good effect on slowing down the corrosion of valves. the
Therefore, the design and manufacturing departments should improve those parts that are unreasonable in structural design, incorrect in process methods, and prone to corrosion to make them suitable for various working conditions. For different corrosion types of valve parts, small There is a unique trick: The methods to prevent intergranular corrosion of austenitic stainless steel valve parts are: to carry out "solution quenching" treatment, that is, to heat to about 1100 ℃ and water quenching, and to choose titanium and niobium, and the carbon content is below 0.03%. Austenitic stainless steel, 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 in welding and cold working through heat treatment, improve the unreasonable valve structure, avoid stress concentration, and adopt electrochemical protection, Spray anti-corrosion coatings, add corrosion inhibitors, 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 kind of corrosion mostly occurs on the sealing surface. Prevention method: choose corrosion-resistant and wear-resistant materials , improve structural design, adopt cathodic protection, etc.
Friction corrosion is two parts that are in contact with each other under load at the same time, and the contact surface is damaged due to vibration and sliding. Friction corrosion occurs at the bolt connection, the connection between the valve stem and the closing member, and between the ball bearing and the shaft. It can be protected by applying lubricating grease, reducing friction, surface phosphating, choosing hard alloy, and using tile spraying treatment or cold working to increase the surface hardness. After welding, corresponding protective measures such as annealing treatment should be adopted as much as possible. 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 gaskets, use flexible graphite and plastic packing, as well as flexible graphite paste gaskets and poly-four Fluorine vinyl gaskets can improve the sealing performance and reduce the corrosion of the valve stem and the sealing surface of the flange.