How protect valves from Corrosion
1. Coating
The coating is the most widely used anti-corrosion method, and it is a kind of anti-corrosion material and identification mark on valves. The coating is also a non-metallic material, which is usually made of synthetic resin, rubber slurry, vegetable oil, solvent, etc. It is covered on the surface of the metal, avoiding contacting the medium and atmosphere to achieve anti-corrosion. The coating is mainly used in environments where water, salt water, seawater, 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 valves are painted, usually every six months to one year.
2. Adding corrosion inhibitor
The mechanism of corrosion inhibitors to control corrosion is that they promote 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. For example, chromium-nickel stainless steel in sulfuric acid not containing oxygen is cremated in a large solubility range, and the corrosion is more serious. However, 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 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.
To protect the valve stem and prevent the corrosion of asbestos packing, in 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. The film can improve the corrosion resistance of the valve stem; the solvent can dissolve the corrosion inhibitor slowly and can also play a role in lubricating; adding zinc powder to asbestos as sacrificial metal zinc is also a corrosion inhibitor. It can combine with the chloride in asbestos so that the chance of contact between the chloride and the metal of the valve stem is greatly reduced 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 atmospheric corrosion.
3. 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. Cathodic protection used for large valves and important valves is an economical simple and effective method. Adding zinc to the asbestos filler to protect the valve stem is also a cathodic protection method.
4. Controlling the corrosive environment
The so-called environment has two kinds, that is, 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. Only in the case of no damage to the product and process, methods of controlling the environment can be used, such as deoxidizing boiler water and adjusting the PH value of home alkali in the refining process. From this point of view, the above-mentioned addition of corrosion inhibitors, and electrochemical protection also belongs 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 clean, purge and lubricate the valve. These are effective measures to control environmental corrosion. Installing a protective cover on the valve stem, installing a well on the ground valve and painting on the surface of the valve are all methods to prevent corrosion of the valve by corrosive substances. Ambient temperature rise and air pollution will accelerate their corrosion, especially for equipment and valves in a closed environment. Outdoor workshops should be used as much as possible or ventilation and cooling measures should be adopted to slow down environmental corrosion.
5. Improving 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 with a reasonable structural design and correct process method will undoubtedly have a good effect on slowing down the corrosion of valves. 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. 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 perform water quenching; choose titanium, niobium, and austenitic stainless steel with a 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 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 wear and corrosion of metal. It is a common corrosion of valves. This kind of corrosion occurs mostly on the sealing surface. Prevention methods: Choose corrosion-resistant and wear-resistant materials; improve structural design and adopt cathodic protection. Friction corrosion is when two parts that are in contact with each other are under load at the same time. The contact surface is damaged due to vibration and sliding. Corrosion caused by friction occurs at the bolt connection, the connection between the valve stem and the closing member, and between the ball bearing and the shaft, which can be protected by applying lubricating grease to reduce friction. Improve surface hardness by spraying tile treatment or cold working. 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 other valve parts. The higher the surface roughness level is, the better the corrosion resistance becomes. Improve the processing technology and structure of packing and gaskets. Using flexible graphite and plastic packing, as well as flexible graphite paste gaskets and PTFE-packed gaskets can improve sealing and reduce corrosion of valve stems and flange sealing surface.
The coating is the most widely used anti-corrosion method, and it is a kind of anti-corrosion material and identification mark on valves. The coating is also a non-metallic material, which is usually made of synthetic resin, rubber slurry, vegetable oil, solvent, etc. It is covered on the surface of the metal, avoiding contacting the medium and atmosphere to achieve anti-corrosion. The coating is mainly used in environments where water, salt water, seawater, 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 valves are painted, usually every six months to one year.
2. Adding corrosion inhibitor
The mechanism of corrosion inhibitors to control corrosion is that they promote 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. For example, chromium-nickel stainless steel in sulfuric acid not containing oxygen is cremated in a large solubility range, and the corrosion is more serious. However, 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 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.
To protect the valve stem and prevent the corrosion of asbestos packing, in 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. The film can improve the corrosion resistance of the valve stem; the solvent can dissolve the corrosion inhibitor slowly and can also play a role in lubricating; adding zinc powder to asbestos as sacrificial metal zinc is also a corrosion inhibitor. It can combine with the chloride in asbestos so that the chance of contact between the chloride and the metal of the valve stem is greatly reduced 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 atmospheric corrosion.
3. 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. Cathodic protection used for large valves and important valves is an economical simple and effective method. Adding zinc to the asbestos filler to protect the valve stem is also a cathodic protection method.
4. Controlling the corrosive environment
The so-called environment has two kinds, that is, 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. Only in the case of no damage to the product and process, methods of controlling the environment can be used, such as deoxidizing boiler water and adjusting the PH value of home alkali in the refining process. From this point of view, the above-mentioned addition of corrosion inhibitors, and electrochemical protection also belongs 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 clean, purge and lubricate the valve. These are effective measures to control environmental corrosion. Installing a protective cover on the valve stem, installing a well on the ground valve and painting on the surface of the valve are all methods to prevent corrosion of the valve by corrosive substances. Ambient temperature rise and air pollution will accelerate their corrosion, especially for equipment and valves in a closed environment. Outdoor workshops should be used as much as possible or ventilation and cooling measures should be adopted to slow down environmental corrosion.
5. Improving 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 with a reasonable structural design and correct process method will undoubtedly have a good effect on slowing down the corrosion of valves. 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. 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 perform water quenching; choose titanium, niobium, and austenitic stainless steel with a 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 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 wear and corrosion of metal. It is a common corrosion of valves. This kind of corrosion occurs mostly on the sealing surface. Prevention methods: Choose corrosion-resistant and wear-resistant materials; improve structural design and adopt cathodic protection. Friction corrosion is when two parts that are in contact with each other are under load at the same time. The contact surface is damaged due to vibration and sliding. Corrosion caused by friction occurs at the bolt connection, the connection between the valve stem and the closing member, and between the ball bearing and the shaft, which can be protected by applying lubricating grease to reduce friction. Improve surface hardness by spraying tile treatment or cold working. 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 other valve parts. The higher the surface roughness level is, the better the corrosion resistance becomes. Improve the processing technology and structure of packing and gaskets. Using flexible graphite and plastic packing, as well as flexible graphite paste gaskets and PTFE-packed gaskets can improve sealing and reduce corrosion of valve stems and flange sealing surface.
