Corrosion Prevention Measures for Metal Valves
Corrosion is one of the main causes of damage to metal valves. The corrosion of metal valves is mainly electrochemical corrosion. In the use of valves, the first consideration of corrosion protection is to eliminate electrochemical corrosion. When electrochemical corrosion can not be eliminated, passivation film should be formed on the metal surface, or non-metallic materials should be used instead of metal materials to control the occurrence of corrosion.
2 Elimination of electrochemical corrosion
2.1 Material Selection
Many mediums are corrosive, and their corrosion principle is very complex. Even if the same valve material is used in the same medium, the corrosion of the medium to the material will be different if the concentration, temperature and pressure of the medium are different. The corrosion rate increases by 1-3 times with the increase of medium temperature by 10 C. Medium concentration also has a great influence on the corrosion of valve materials. If lead is in low concentration sulfuric acid, the corrosion is very small. When the concentration exceeds 96%, the corrosion rises sharply. The corrosion of carbon steel is the most serious when the sulfuric acid concentration is about 50%. When the sulfuric acid concentration increases to more than 60%, the corrosion slows down. Aluminum has strong corrosion resistance in concentrated nitric acid with a concentration of more than 80%, but it is seriously corroded in medium and low concentration nitric acid. Although stainless steel has strong corrosion resistance to dilute nitric acid, its corrosion is aggravated in more than 95% concentrated nitric acid. Therefore, according to the specific situation, various factors affecting corrosion should be analyzed, and valve materials should be selected correctly.
2.2 Improvement of process and structure
The anti-corrosion protection of the valve should be considered from the design. If the structure of the valve is reasonable and the processing method is correct, the corrosion of the valve can be greatly reduced. Therefore, the parts which are easy to cause corrosion in valves should be improved to meet the requirements of different working conditions. If the gap in the valve connection can cause oxygen concentration battery corrosion, therefore, the connection between the stem and the closing parts should not be threaded as far as possible. Spot welding and lap welding are prone to corrosion. Double-sided welding and continuous welding should be adopted for valve welding. Polytetrafluoroethylene (PTFE) raw material belt should be used at the threaded connection of the valve, which not only has good sealing, but also can prevent corrosion. Where the medium in the valve is not easy to flow, it is easy to be corroded. In addition to discharging sedimentary medium when installing and using the valve, the sag structure should be avoided as far as possible when manufacturing the valve parts, and the discharge holes should be set as far as possible. Different metal contacts will form galvanic couples and promote corrosion of anodic metals. When selecting materials, attention should be paid to avoid metal contacts with large potential difference and no passivation film. Stress corrosion will occur in the process of welding and heat treatment. Attention should be paid to improving the processing methods. After welding, the corresponding protective measures such as annealing treatment should be adopted as far as possible. Improve the surface finish grade of the stem and other valve parts, with good surface finish and corrosion resistance. Improve the processing technology and structure of fillers and gaskets, using flexible graphite, plastic fillers, flexible graphite sticking gaskets and PTFE gaskets, not only improve sealing performance, but also reduce the corrosion of valve stem and flange sealing surface.
2.3 Cathodic Protection
Cathodic protection is to use the protected metal as cathode and direct current to reduce its potential to negative direction. When it reaches a certain potential, the corrosion current decreases rapidly and the metal is protected. In addition, cathodic protection can protect the protected metal with a lower electrode potential than that of the protected metal. If zinc is used to protect iron, zinc is corroded. Zinc is called sacrificial metal. Cathodic protection is an economical, simple and effective method for large valves and important valves. Zinc protection valve stem added to asbestos packing belongs to cathodic protection method.
3 Surface Passivation Film Treatment
3.1 Adding Corrosion Inhibitor
Adding a small amount of other special substances in corrosive media and corrosive substances can greatly slow down the corrosion rate of metals. This special substance is called corrosion inhibitor. The mechanism of corrosion inhibition is that it promotes the polarization of batteries. Corrosion inhibitors are mainly used in media and fillers. The corrosion of equipment and valves can be slowed down by adding corrosion inhibitors in the medium. For example, chromium-nickel stainless steel in oxygen-free sulphuric acid forms an active state in a large concentration range, and the corrosion is serious. However, after adding a small amount of oxidant such as copper sulphate or nitric acid, the stainless steel can be turned into passive state, and a protective film is formed on the surface to prevent the introduction. Quality erosion. In hydrochloric acid, if a small amount of oxidant is added, the corrosion of titanium can be reduced. Valves often use water as pressure test medium, which is easy to cause valve corrosion. Adding a small amount of sodium nitrite in water can prevent the corrosion of water to the valve.
The asbestos packing contains chloride, which corrodes the valve stem greatly. The chloride content can be reduced by washing with distilled water. However, this method is difficult to implement and should not be popularized widely. It is only suitable for special needs. In order to protect the valve stem from corrosion of asbestos packing, corrosion inhibitors and sacrificial metals are coated in the asbestos packing and on the valve stem. The corrosion inhibitor is composed of sodium nitrite, sodium chromate and solvent. Sodium nitrite and sodium chromate can form a passivation film on the surface of the valve stem and improve the corrosion resistance of the valve stem. Solvents can dissolve corrosion inhibitors slowly and lubricate them. Zinc powder is added to asbestos as sacrificial metal. In essence, zinc is also a corrosion inhibitor. It can first combine with chloride in asbestos, so that chloride and valve stem metal contact opportunities are reduced, so as to achieve the purpose of corrosion protection. If some corrosion inhibitors such as red red red red and calcium lead are added to the coating, spraying on the surface of the valve can prevent atmospheric corrosion.
3.2 Surface Coating
Metal surface treatment technology includes surface coating, surface penetration, surface oxidation passivation and so on. Its purpose is to improve the corrosion resistance of metals and the mechanical properties of metals. Surface treatment is widely used in valves. Valve connection bolts are commonly treated with galvanizing, chromium plating and oxidation (bluing) to improve their resistance to atmospheric or medium corrosion. In addition to the above methods, other fasteners can be treated by surface treatment processes such as phosphating and passivation according to the situation. Surface treatment processes such as nitriding or boronizing are often used to improve the corrosion resistance and wear resistance of sealing surfaces and closed parts with small caliber. If the valve disc is made of 38CrMoAlA, the nitriding layer thickness is more than 0.4 mm.
The valve stem is usually treated by nitriding, boronizing, chromium plating and nickel plating to improve its corrosion resistance, wear resistance and abrasion resistance.
Different surface treatment is suitable for different material and working environment of valve stem. Hard chromium plating and gas nitriding process can be used for valve stem contacting with asbestos filler in air or steam medium (ion nitriding process is not suitable for stainless steel). In hydrogen sulfide gas environment, high phosphorus nickel plating has better protection performance. Ion and gas nitriding can improve corrosion resistance of 38CrMoAlA, but hard chromium coating is not suitable. 2Cr13 can resist ammonia corrosion after quenching and tempering, and carbon steel nitrided by gas can also resist ammonia corrosion, but all phosphorus-nickel coatings are not resistant to ammonia corrosion. 38CrMoAlA material nitrided by gas has excellent corrosion resistance and comprehensive performance, and is often used to make valve stem.
Small caliber valves and handwheels are often chrome plated to improve their corrosion resistance and decorate valves.
3.3 Anode Protection
Anodic protection is to introduce the protected metal as the anode into the external direct current, so that the anode potential increases in a positive direction. When it increases to a certain value, a dense protective film is formed on the surface of the metal anode, i.e. passivation film. At this time, the corrosion of the metal anode decreases sharply. Anodic protection is suitable for metals that are easily passivated. In production practice, less anode protection is used and more cathodic protection is used.
4.1 Nonmetallic Materials
Non-metallic materials have excellent corrosion resistance. As long as the temperature and pressure of valves meet the requirements of non-metallic materials, the use of non-metallic materials can not only solve the problem of corrosion resistance, but also save precious metals and reduce the cost of valves. The body and cover of the valve are usually made of cast iron or carbon steel. Polytetrafluoroethylene, chlorinated polyether, natural rubber, chloroprene rubber and nitrile-butadiene rubber are used as the lining of the valve, which not only ensures the strength of the valve, but also ensures that the valve is not corroded. Gaskets and fillers are mainly made of non-metallic materials. The pinch valve is also designed according to the excellent corrosion resistance and deformation performance of rubber. Nowadays, more and more valves use nylon, polytetrafluoroethylene and other plastics as well as natural rubber and synthetic rubber to make various sealing surfaces and rings. These non-metallic materials have good corrosion resistance and sealing performance, and are especially suitable for use in medium with particles. However, because of its low strength and heat resistance, its application scope is limited. Flexible graphite makes non-metallic materials enter the high temperature field, solves the problem of leakage of fillers and gaskets which is difficult to solve for a long time, and is a good high temperature lubricant.
4.2 Spray Paint
Coatings are one of the most widely used anti-corrosion means, and they are indispensable anti-corrosion materials and identification marks in valve products. Coatings are usually made of synthetic resin, rubber slurry, vegetable oil, solvents, etc. They cover the metal surface, isolate the medium and atmosphere, and achieve the purpose of anti-corrosion. Coatings are mainly used in water, brine, seawater or atmospheric environments where corrosion is not strong. Anti-corrosive paint is commonly used in valve chamber to prevent water and air from corroding the valve. The paint is mixed with different colors to represent the material of the valve.
5 Other measures
5.1 Thermal Spraying
Thermal spraying is a process for preparing coatings and has become one of the new technologies for surface protection and strengthening of materials. Thermal spraying is the use of high energy density heat sources (gas combustion flame, arc, plasma arc, electrothermal, gas explosion, etc.) to melt metal or non-metallic materials and spray them to the pretreated substrate surface in the form of atomization, forming spray coating, or heating the substrate surface at the same time, so that the coating on the substrate surface. Surface hardening process for forming spray welding layer by melting again. Most metals and their alloys, metal oxide ceramics, metal ceramic composites and hard metal compounds can be coated on metal or non-metal substrates by one or more thermal spraying methods. Thermal spraying can improve the surface corrosion resistance, wear resistance, high temperature resistance and other properties, prolong the service life. Thermal spraying special functional coatings, with heat insulation, insulation (or conductivity), sealing, self-lubrication, thermal radiation and electromagnetic shielding and other special properties. Components can also be repaired by thermal spraying.
5.2 Environmental Control
The atmosphere is full of dust, steam and smoke. Especially in the production environment, toxic gases and dust emitted from chimneys and equipment will cause varying degrees of corrosion to valves. The valves should be cleaned and purged regularly and refueled regularly according to the rules of operation, which is an effective measure to control environmental corrosion. Valve stem mounting protective cover, ground valve mounting wells and valve surface painting are also effective ways to prevent corrosive substances from corroding valves. Rising ambient temperature and air pollution will accelerate the corrosion of equipment and valves in closed environment. Open workshop or ventilation measures should be adopted to reduce environmental corrosion as far as possible.
6 Concluding remarks
Anti-corrosion of metal valves is a systematic project. From the design, manufacture, management and use of valves, this problem is involved in every link. Only by solving this problem carefully can the optimization of anti-corrosion of metal valves be realized.
Source: China Valves Manufacturer – Yaang Pipe Industry Co., Limited (www.ugsteelmill.com)
(Yaang Pipe Industry is a leading manufacturer and supplier of nickel alloy and stainless steel products, including Super Duplex Stainless Steel Flanges, Stainless Steel Flanges, Stainless Steel Pipe Fittings, Stainless Steel Pipe. Yaang products are widely used in Shipbuilding, Nuclear power, Marine engineering, Petroleum, Chemical, Mining, Sewage treatment, Natural gas and Pressure vessels and other industries.)
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