Metal valves can be said to be the key component structures that are 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 used, and the valve body and valve cover are suitable for two or three. Materials, valves used for 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, and fatigue corrosion at local locations of the metal. , Selective corrosion, wear corrosion, cavitation corrosion, friction corrosion, hydrogen corrosion and other local 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 the medium is different in the concentration, temperature and pressure of the medium, the corrosion of the medium on the material is also different. When the temperature of the medium increases by 10°C, 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 rises sharply. Contrary to carbon steel, the corrosion is severe when the concentration of sulfuric acid 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 corroded in medium and low concentrations of nitric acid. 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. Use 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. Valve body, bonnet, lining, sealing surface, etc. are commonly made of non-metallic materials. As for gaskets, packings are mainly made of non-metallic materials. The valve lining is made of plastics such as polytetrafluoroethylene and chlorinated polyether, as well as rubber such as natural rubber, neoprene and nitrile rubber, and the valve body and valve cover are made of 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 good corrosion resistance and mutation performance of rubber. Nowadays, it is more and more correct to use nylon, PTFE and other plastics, and natural rubber and synthetic rubber to make various sealing surfaces and sealing rings, which are used on various types 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 medium with particles. Of course, their strength and heat resistance are low, limiting the range of applications. The emergence of flexible graphite has brought non-metals into the high temperature domain, solved the long-term difficult to solve the problem of packing and gasket leakage, and is a good high temperature lubricant.
3. Metal surface treatment
The valve connecting screw is usually galvanized, chrome-plated, and oxidized (blued) to improve the resistance to 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 adopt surface processes such as nitriding and boronizing to improve its corrosion resistance and wear resistance. The valve disc made of 38CrMoAlA, the nitrided 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 working environments. The valve stem in contact with the atmosphere, water vapor medium and asbestos packing can be plated with hard chrome and gas nitriding process (stainless steel is not suitable for ion nitriding process); In the atmosphere of hydrogen sulfide, the valve is electroplated with high phosphorus nickel coating, which has better 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. Carbon steel using gas nitriding can also resist ammonia corrosion, while all phosphorus-nickel coatings are not resistant to ammonia corrosion; after gas nitriding, 38CrMoAlA material has good 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 kind of process block for preparing coatings and has become one of the new technologies for material surface protection. It uses a high energy density heat source (gas combustion flame, electric arc, plasma arc, electric heat, gas explosion, etc.) to heat and melt metal or non-metallic materials, and sprays them onto the pretreated basic surface in the form of atomization to form a spray coating. , or heating the basic surface at the same time, so that the coating is melted again on the surface of the substrate, and the surface strengthening process of the spray welding layer is formed. 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 spray 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 coating has special properties such as heat insulation, edge (or different electricity), grindable sealing, self-lubricating, heat radiation, electromagnetic shielding, etc.; parts can be repaired by thermal spraying.
5. Spray paint
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. They are usually made of synthetic resin, rubber slurry, vegetable oil, solvent, etc., and cover the metal surface to isolate the medium and the atmosphere to achieve anti-corrosion purposes. Coatings are mainly used in environments that are not very corrosive, such as water, salt water, seawater, and atmosphere. 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 colours to represent the materials Fahn uses.
6. Add corrosion inhibitor
The mechanism by which the corrosion inhibitor controls corrosion is that it promotes the transformation of the battery. Corrosion inhibitors are mainly used in media and fillers. Adding a corrosion inhibitor to the medium can slow down the corrosion of equipment and valves. For example, in oxygen-free sulfuric acid, chromium-nickel stainless steel becomes cremated in a wide range of solubility, and the corrosion is more serious, but a small amount of copper sulfate or nitric acid is added. When the oxidant is used, the stainless steel can be transformed into a passive state, and a protective film is formed on the surface to prevent the corrosion 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. However, this method is difficult to implement and cannot be generally promoted, and is suitable for special needs.
In order to protect the valve stem and prevent the corrosion of the asbestos packing, the valve stem is filled with corrosion inhibitor and sacrificial metal in the asbestos packing. The corrosion inhibitor is composed of sodium nitrite and sodium chromate, which can form a passivation film on the surface of the valve stem and improve the corrosion resistance of the valve stem; the solvent can slowly dissolve the corrosion inhibitor and play a lubricating role; in asbestos Zinc powder is added as a sacrificial metal. In fact, zinc is also a kind of corrosion inhibitor. It can first combine with the chloride in asbestos, so that the contact opportunity between the chloride and the valve stem metal is greatly reduced, so as to achieve the purpose of anti-corrosion. If a corrosion inhibitor such as red red and calcium lead acid is added to the paint, spraying on the surface of the valve can prevent atmospheric corrosion.
7. Electrochemical protection
There are two types of electrochemical protection: positive protection and negative protection. If zinc is used to protect iron, zinc is corroded, and zinc is called a sacrificial metal. In production practice, yang protection is used less, and yin protection is used more. Large valves and important valves use this negative protection method, which is an economical, simple and effective method. Zinc is added to the asbestos filler to protect the valve stem, which also belongs to the negative protection method.
8. Control the corrosive environment
The so-called environment has two broad senses and narrow sense. 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 production processes cannot be changed arbitrarily. Only in the case that it will not cause damage to products and processes, environmental control methods can be used, such as boiler water deoxygenation, alkali adjustment of PH value in oil refining process, etc. From this point of view, the above-mentioned addition of corrosion inhibitors, electrochemical protection, etc. are also controlled corrosion 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 equipment, which will corrode the valve to varying degrees. Operators should regularly clean and purge valves and refuel regularly according to the regulations in the operating procedures, which are effective measures to control environmental corrosion. The valve stem is installed with a protective cover, the ground valve is installed in the ground well, and the valve surface is sprayed with paint, etc., which are all methods to prevent the corrosion of the valve from containing corrosive substances. Elevated ambient temperature and air pollution, especially for equipment and valves in closed environments, will accelerate their corrosion. Open workshops or ventilation and cooling measures should be adopted as far 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 reasonable structural design and correct process method. Undoubtedly, it has a good effect on slowing down the corrosion of the valve.
Therefore, the design and manufacturing departments should improve the parts that are unreasonable in structural design, incorrect in process methods, and prone to corrosion, so as to meet the requirements of various working conditions.