Several Ways to Resist Corrosion of Valves
Jun 25, 2023
Metal valves can be said to be the part that is 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, less corrosive materials are used.
Corrosion of valves
Under complex working conditions such as atmosphere or solution, metal valves not only undergo uniform corrosion on the metal surface all the time, pitting corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, abrasion corrosion, cavitation corrosion, friction corrosion and hydrogen corrosion are also prone to occur.
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 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 to 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 little. 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 good 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 for non-metallic materials, it can not only solve the corrosion but also save precious metals. The valve body, bonnet, lining and sealing surface of the valve are usually made from non-metallic materials. 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 bonnet 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, which not only have good corrosion resistance, but also good sealing performance, especially suitable for 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 the high-temperature field, which 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. Surface treatment of metal
The connection of the valve: screws connected to bolts 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 their corrosion resistance and wear resistance. The valve disc made of 38CrMoAlA has a nitrided layer greater than and equal to 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 working environments. The valve stem in contact with the asbestos filler in the atmosphere and water vapor medium can be plated with hard chromium and gas nitriding process (stainless steel is not suitable for ion nitriding process); the valve in the atmosphere of hydrogen sulfide can be plated with high phosphorus nickel, which has good protective performance for coatings; 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. All phosphorus-nickel coatings are not resistant to ammonia corrosion; 38CrMoAlA material after gas nitriding has excellent corrosion resistance and comprehensive performance, and it is mostly used to make valve stems. The valve body with small diameters and hand wheels are also often chrome plated to improve their corrosion resistance and decorate the valve.
4. Thermal spraying
Thermal spraying is a kind of process for preparing coatings, and has become one of the new technologies for material surface protection. It uses high energy density heat sources such as gas combustion flames, electric arcs, plasma arcs, electric heating and gas explosions to spray metal or non-metallic material that is heated and melted onto the pretreated basic surface in the form of atomization to form a spray coating. 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, and high temperature resistance and prolong its service life. Thermal spraying special functional coatings have special properties such as heat insulation, insulation, wearable seals, self-lubrication, heat radiation, and special properties such as electromagnetic shielding; parts can be repaired by thermal spraying.
Corrosion of valves
Under complex working conditions such as atmosphere or solution, metal valves not only undergo uniform corrosion on the metal surface all the time, pitting corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, abrasion corrosion, cavitation corrosion, friction corrosion and hydrogen corrosion are also prone to occur.
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 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 to 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 little. 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 good 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 for non-metallic materials, it can not only solve the corrosion but also save precious metals. The valve body, bonnet, lining and sealing surface of the valve are usually made from non-metallic materials. 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 bonnet 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, which not only have good corrosion resistance, but also good sealing performance, especially suitable for 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 the high-temperature field, which 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. Surface treatment of metal
The connection of the valve: screws connected to bolts 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 their corrosion resistance and wear resistance. The valve disc made of 38CrMoAlA has a nitrided layer greater than and equal to 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 working environments. The valve stem in contact with the asbestos filler in the atmosphere and water vapor medium can be plated with hard chromium and gas nitriding process (stainless steel is not suitable for ion nitriding process); the valve in the atmosphere of hydrogen sulfide can be plated with high phosphorus nickel, which has good protective performance for coatings; 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. All phosphorus-nickel coatings are not resistant to ammonia corrosion; 38CrMoAlA material after gas nitriding has excellent corrosion resistance and comprehensive performance, and it is mostly used to make valve stems. The valve body with small diameters and hand wheels are also often chrome plated to improve their corrosion resistance and decorate the valve.
4. Thermal spraying
Thermal spraying is a kind of process for preparing coatings, and has become one of the new technologies for material surface protection. It uses high energy density heat sources such as gas combustion flames, electric arcs, plasma arcs, electric heating and gas explosions to spray metal or non-metallic material that is heated and melted onto the pretreated basic surface in the form of atomization to form a spray coating. 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, and high temperature resistance and prolong its service life. Thermal spraying special functional coatings have special properties such as heat insulation, insulation, wearable seals, self-lubrication, heat radiation, and special properties such as electromagnetic shielding; parts can be repaired by thermal spraying.
