Corrosion is the destruction and deterioration of materials under the action of various environments. The corrosion of metal is mainly caused by chemical corrosion and pitting chemical corrosion, and the corrosion of non-metallic materials is generally caused by direct chemical and physical damage.
The forms of the valve's corrosion
There are two forms for corrosion of metal valves, namely uniform corrosion and localized corrosion. The rate of uniform corrosion can be evaluated by the average annual corrosion rate. There are four grades for metal materials, graphite, glass, ceramics, and concrete according to the corrosion rate. The corrosion rate which is less than 0.05mm/a is excellent; the corrosion rate that is between 0.05 and 0.5mm/a is good; the corrosion rate from 0.5 to 1.5mm/a can still be used; the corrosion rate greater than 1.5mm/a is not applicable. First-class materials are usually adopted for valve parts such as sealing surfaces of valves, valve stems, diaphragm, small springs, and other valve parts, while second-class or third-class materials are suitable for valve bodies and bonnets. As for valves with high pressure, highly toxic, flammable, explosive, and radioactive media, materials with little corrosiveness should be used.
1. Uniform corrosion
Uniform corrosion is performed on the entire surface of the metal. For example, a protective film is generated in an oxidizing environment for metal such as stainless steel, aluminum, titanium, etc., and the metal under the film will corrode uniformly. There is also a phenomenon, that is corrosion and peeling of metal surfaces. This kind of corrosion is the most dangerous.
2. Localized corrosion
Localized corrosion occurs on the partial metal, which has forms like pitting corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, wear corrosion, cavitation corrosion, friction and vibration corrosion, hydrogen corrosion, etc.
Pitting corrosion
Pitting corrosion usually occurs on the metal which has the passivation or protective film. Because of the defects on the metal surface, the active ions in the passivation film can be destroyed by the solution, causing the passivation film to be partially destroyed, extending into the metal and becoming pitting corrosion. It is one of the most destructive and hidden corrosion forms of metals.
Crevice corrosion
Crevice corrosion happens in welding, riveting, gasket, and deposit environments, which is a special form for pitting corrosion. The prevention method is to eliminate the gap.
Intergranular corrosion starts from the surface, then goes along the grain boundary to the inner metal, thus causing the grain boundary to be corroded and showing corrosion shaped like a net. In addition to impurities deposited on grain boundaries, intergranular corrosion is mainly caused by improper heat treatment and cold working. Austenitic stainless steel is prone to chromium depleted areas on both sides of the welded seam and is corroded. Intergranular corrosion of austenitic stainless steel is the most common and dangerous form of corrosion. Methods to prevent intergranular corrosion of valve parts in austenitic stainless steel are as follows. Perform solution hardening, that is, heating to about 1100℃ and then water quenching; select austenitic stainless steel containing titanium and niobium and carbon content below 0.03% to reduce the generation of chromium carbide.
Delamination corrosion
Delamination corrosion generates in a layered structure. The corrosion first develops vertically and inwardly and then corrodes the parallel material on the surface. Under the expansion force of the corroded material, the surface is peeled off in layers.
Stress corrosion
Stress corrosion is a kind of crack that happens under the simultaneous action of corrosion and tensile stress. Methods to prevent stress corrosion: eliminate or reduce the stress generated in welding and cold working through heat treatment; improve the unreasonable valve structure; avoid stress concentration; use electrochemical protection and spray anti-corrosion paint; add corrosion inhibitors and apply compressive stress.
Corrosion fatigue
Corrosion fatigue occurs in the joint action of alternating stress corrosion, causing the metal to crack. Heat treatment can be conducted to eliminate or reduce stress. Shot blasting, electroplating of zinc, chromium and nickel on the surface can be applied, but it should be noted that there should not be tensile stress and hydrogen diffusion for the plating layer.
Selective corrosion
Selective corrosion occurs in materials with different compositions and impurities. In a certain environment, some elements are corroded and leached, and the remaining uncorroded elements are sponge-like. Dezincification of brass, dealumination of copper alloys and graphitization of cast iron are common to be seen.
Abrasion corrosion
Abrasion corrosion is a form of corrosion produced by the alternating action of the fluid on metal wear and corrosion, which is a common type of corrosion in valves. This type of corrosion occurs mostly on the sealing surface. Prevention methods are: use corrosion resistant and wear-resistant materials; improve the structural design and adopt cathodic protection.
Cavitation corrosion
Cavitation corrosion is a special form of wear corrosion. It is a bubble generated in the fluid, and a shock wave generates when it bursts. The pressure can be as high as 400 atmospheres, which can destroy the metal's protective film and even tear the metal particles. Then a film is formed due to corrosion. This process is repeated continuously to corrode the metal. This process is repeated continuously to corrode the metal. Choosing cavitation corrosion-resistant materials, highly polished processing surface, elastic protective layers and cathodic protection can prevent cavitation corrosion
Friction and vibration corrosion
Friction and vibration corrosion is the damage caused by vibration and sliding on the contact surface when two parts in contact with each other bear the load at the same time. Friction and vibration corrosion happens at the connection part of bolts, valve stems and closures, ball bearing and shafts. Adopt lubricating grease, surface phosphating, cemented carbide, shot blasting and cold working to improve surface hardness.
Corrosion is the damage caused by the diffusion of hydrogen atoms generated in chemical reactions in the metal. Its forms include hydrogen bubbling, hydrogen embrittlement, and hydrogen corrosion. Strong steel and steel containing non-metal are prone to hydrogen bubbling. When petroleum contains sulfides and hydrides, hydrogen bubbling is likely to occur. Using void-free killed steel instead of voided boiling steel, rubber and plastic for protection and adding corrosion inhibitors to prevent bubbling. Using void-free killed steel instead of voided boiling steel, rubber and plastic for protection and adding corrosion inhibitors to prevent bubbling. Alloy steels containing nickel and lead should be used, while high-strength steel with much hydrogen embrittlement should be avoided. Hydrogen embrittlement should be avoided or reduced during welding, electroplating, and pickling. Under high temperature and high pressure, hydrogen enters the metal, and reacts with one element, which is called hydrogen corrosion. Austenitic stainless steel is fully resistant to high temperature hydrogen corrosion.
Non-metal corrosion has the same corrosion size as metal corrosion. Most non-metal materials are non-electrical conductors and generally do not produce electrochemical corrosion, but pure chemical or physical corrosion. This is the main difference from metal corrosion. Non-metallic corrosion is not necessarily weight loss but often weight gain. For metal corrosion, weight loss is the main feature. Many non-metal corrosions are caused by physical effects, and the physical effects of metal corrosion are very rare; non-metal internal corrosion is common to be seen. Metal corrosion mainly features surface corrosion.
After the metal comes into contact with the medium, the solution or gas will gradually diffuse into the inside of the material, causing a series of corrosion changes to the non-metallic material. Depending on the type of the non-metallic material, the form of corrosion varies. The forms of corrosion include dissolution, swelling, bubbling, softening, decomposition, discoloration, deterioration, aging, hardening, and fracturing. However, from a comprehensive point of view, the corrosion performance of non-metallic materials is much better than that of metallic materials, while the strength and temperature resistance of non-metallic materials are worse than those of metallic materials.
The forms of the valve's corrosion
There are two forms for corrosion of metal valves, namely uniform corrosion and localized corrosion. The rate of uniform corrosion can be evaluated by the average annual corrosion rate. There are four grades for metal materials, graphite, glass, ceramics, and concrete according to the corrosion rate. The corrosion rate which is less than 0.05mm/a is excellent; the corrosion rate that is between 0.05 and 0.5mm/a is good; the corrosion rate from 0.5 to 1.5mm/a can still be used; the corrosion rate greater than 1.5mm/a is not applicable. First-class materials are usually adopted for valve parts such as sealing surfaces of valves, valve stems, diaphragm, small springs, and other valve parts, while second-class or third-class materials are suitable for valve bodies and bonnets. As for valves with high pressure, highly toxic, flammable, explosive, and radioactive media, materials with little corrosiveness should be used.
1. Uniform corrosion
Uniform corrosion is performed on the entire surface of the metal. For example, a protective film is generated in an oxidizing environment for metal such as stainless steel, aluminum, titanium, etc., and the metal under the film will corrode uniformly. There is also a phenomenon, that is corrosion and peeling of metal surfaces. This kind of corrosion is the most dangerous.
2. Localized corrosion
Localized corrosion occurs on the partial metal, which has forms like pitting corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, wear corrosion, cavitation corrosion, friction and vibration corrosion, hydrogen corrosion, etc.
Pitting corrosion
Pitting corrosion usually occurs on the metal which has the passivation or protective film. Because of the defects on the metal surface, the active ions in the passivation film can be destroyed by the solution, causing the passivation film to be partially destroyed, extending into the metal and becoming pitting corrosion. It is one of the most destructive and hidden corrosion forms of metals.
Crevice corrosion
Crevice corrosion happens in welding, riveting, gasket, and deposit environments, which is a special form for pitting corrosion. The prevention method is to eliminate the gap.
Intergranular corrosion starts from the surface, then goes along the grain boundary to the inner metal, thus causing the grain boundary to be corroded and showing corrosion shaped like a net. In addition to impurities deposited on grain boundaries, intergranular corrosion is mainly caused by improper heat treatment and cold working. Austenitic stainless steel is prone to chromium depleted areas on both sides of the welded seam and is corroded. Intergranular corrosion of austenitic stainless steel is the most common and dangerous form of corrosion. Methods to prevent intergranular corrosion of valve parts in austenitic stainless steel are as follows. Perform solution hardening, that is, heating to about 1100℃ and then water quenching; select austenitic stainless steel containing titanium and niobium and carbon content below 0.03% to reduce the generation of chromium carbide.
Delamination corrosion
Delamination corrosion generates in a layered structure. The corrosion first develops vertically and inwardly and then corrodes the parallel material on the surface. Under the expansion force of the corroded material, the surface is peeled off in layers.
Stress corrosion
Stress corrosion is a kind of crack that happens under the simultaneous action of corrosion and tensile stress. Methods to prevent stress corrosion: eliminate or reduce the stress generated in welding and cold working through heat treatment; improve the unreasonable valve structure; avoid stress concentration; use electrochemical protection and spray anti-corrosion paint; add corrosion inhibitors and apply compressive stress.
Corrosion fatigue
Corrosion fatigue occurs in the joint action of alternating stress corrosion, causing the metal to crack. Heat treatment can be conducted to eliminate or reduce stress. Shot blasting, electroplating of zinc, chromium and nickel on the surface can be applied, but it should be noted that there should not be tensile stress and hydrogen diffusion for the plating layer.
Selective corrosion
Selective corrosion occurs in materials with different compositions and impurities. In a certain environment, some elements are corroded and leached, and the remaining uncorroded elements are sponge-like. Dezincification of brass, dealumination of copper alloys and graphitization of cast iron are common to be seen.
Abrasion corrosion
Abrasion corrosion is a form of corrosion produced by the alternating action of the fluid on metal wear and corrosion, which is a common type of corrosion in valves. This type of corrosion occurs mostly on the sealing surface. Prevention methods are: use corrosion resistant and wear-resistant materials; improve the structural design and adopt cathodic protection.
Cavitation corrosion
Cavitation corrosion is a special form of wear corrosion. It is a bubble generated in the fluid, and a shock wave generates when it bursts. The pressure can be as high as 400 atmospheres, which can destroy the metal's protective film and even tear the metal particles. Then a film is formed due to corrosion. This process is repeated continuously to corrode the metal. This process is repeated continuously to corrode the metal. Choosing cavitation corrosion-resistant materials, highly polished processing surface, elastic protective layers and cathodic protection can prevent cavitation corrosion
Friction and vibration corrosion
Friction and vibration corrosion is the damage caused by vibration and sliding on the contact surface when two parts in contact with each other bear the load at the same time. Friction and vibration corrosion happens at the connection part of bolts, valve stems and closures, ball bearing and shafts. Adopt lubricating grease, surface phosphating, cemented carbide, shot blasting and cold working to improve surface hardness.
Corrosion is the damage caused by the diffusion of hydrogen atoms generated in chemical reactions in the metal. Its forms include hydrogen bubbling, hydrogen embrittlement, and hydrogen corrosion. Strong steel and steel containing non-metal are prone to hydrogen bubbling. When petroleum contains sulfides and hydrides, hydrogen bubbling is likely to occur. Using void-free killed steel instead of voided boiling steel, rubber and plastic for protection and adding corrosion inhibitors to prevent bubbling. Using void-free killed steel instead of voided boiling steel, rubber and plastic for protection and adding corrosion inhibitors to prevent bubbling. Alloy steels containing nickel and lead should be used, while high-strength steel with much hydrogen embrittlement should be avoided. Hydrogen embrittlement should be avoided or reduced during welding, electroplating, and pickling. Under high temperature and high pressure, hydrogen enters the metal, and reacts with one element, which is called hydrogen corrosion. Austenitic stainless steel is fully resistant to high temperature hydrogen corrosion.
Non-metal corrosion has the same corrosion size as metal corrosion. Most non-metal materials are non-electrical conductors and generally do not produce electrochemical corrosion, but pure chemical or physical corrosion. This is the main difference from metal corrosion. Non-metallic corrosion is not necessarily weight loss but often weight gain. For metal corrosion, weight loss is the main feature. Many non-metal corrosions are caused by physical effects, and the physical effects of metal corrosion are very rare; non-metal internal corrosion is common to be seen. Metal corrosion mainly features surface corrosion.
After the metal comes into contact with the medium, the solution or gas will gradually diffuse into the inside of the material, causing a series of corrosion changes to the non-metallic material. Depending on the type of the non-metallic material, the form of corrosion varies. The forms of corrosion include dissolution, swelling, bubbling, softening, decomposition, discoloration, deterioration, aging, hardening, and fracturing. However, from a comprehensive point of view, the corrosion performance of non-metallic materials is much better than that of metallic materials, while the strength and temperature resistance of non-metallic materials are worse than those of metallic materials.
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