Causes of valve corrosion and preventive measures

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Corrosion is one of the major factors causing valve damage; therefore, corrosion protection is a key consideration when using valves. Corrosion is the degradation and deterioration of materials under the influence of various environments. The corrosion of metals is mainly caused by chemical corrosion and pitting corrosion, while the corrosion of non-metallic materials is generally resulting from direct chemical and physical actions. I. Forms of valve corrosion There are two forms of corrosion in metal valves, namely uniform corrosion and local corrosion. The rate of uniform corrosion can be evaluated using the annual average corrosion rate. Metal materials, graphite, glass, ceramics, and concrete are classified into four grades based on their corrosion rates: those with a corrosion rate of less than 0.05 mm/year are considered excellent; those with a corrosion rate between 0.05 and 0.5 mm/year are deemed good; those with a corrosion rate between 0.5 and 1.5 mm/year are still acceptable for use; and those with a corrosion rate exceeding 1.5 mm/year are unsuitable for use. For valve components such as sealing surfaces, stems, diaphragms, and small springs, materials of Grade 1 are generally used. Materials of Grade 2 or 3 are suitable for valve bodies and covers. For valves intended to handle high-pressure, highly toxic, flammable, explosive, or radioactive media, materials with very low corrosivity should be selected. 1. Uniform corrosion Uniform corrosion occurs over the entire surface of the metal. Such as the protective layer that forms in an oxidizing environment on metals like stainless steel, aluminum, and titanium; the metal beneath this layer corrodes evenly. There is another phenomenon, namely the corrosion and flaking of metal surfaces; this type of corrosion is the most dangerous. 2. Local corrosion occurs in specific areas of a metal. Its forms include pitting corrosion, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, wear corrosion, cavitation corrosion, erosion-corrosion fatigue, and hydrogen corrosion. Pitting corrosion typically occurs on metals with a passive or protective film. It is caused by defects on the metal surface and active ions in the solution that can destroy the passive film. This leads to localized damage of the film, allowing corrosion to penetrate into the interior of the metal and form pits. It is one of the most destructive and hazardous forms of corrosion for metals. Crevice corrosion occurs in environments such as beneath welds, rivets, gaskets, or deposits; it is a special form of pitting corrosion. The prevention method is to eliminate gaps. Intergranular corrosion progresses from the surface along the grain boundaries deep into the metal, causing the grain boundaries to suffer network-like corrosion. Intergranular corrosion occurs mainly due to improper heat treatment and cold working, in addition to impurities deposited at the grain boundaries. On both sides of the welds in austenitic stainless steel, chromium-depleted zones are prone to forming, leading to corrosion. Intergranular corrosion of austenitic stainless steel is a common and most dangerous form of corrosion. Methods to prevent intergranular corrosion in austenitic stainless steel valve components include performing \"solution annealing,\" that is, heating to around 1100°C followed by quenching in water; using austenitic stainless steels that contain titanium and niobium with a carbon content of less than 0.03%, in order to reduce the formation of chromium carbides. Delamination corrosion occurs in layered structures; the corrosion first progresses vertically inward, and then it affects the materials parallel to the surface. Under the expansion force of the corrosive products, the surface peels off in layers. Stress corrosion occurs as cracking resulting from the combined effect of corrosion and tensile stress. Methods to prevent stress corrosion: eliminating or reducing stresses generated during welding and cold working through heat treatment; improving the structure of valves to prevent stress concentration; employing electrochemical protection; applying anti-corrosion coatings. Measures such as adding corrosion inhibitors and applying compressive stress. Corrosion fatigue occurs at locations where alternating stress and corrosion act together, causing the metal to fracture. Heat treatment can be performed to eliminate or reduce stress. Surface shot peening as well as electroplating with zinc, chromium, nickel, etc., are also possible; however, it is important to ensure that no tensile stress or hydrogen diffusion occurs in the plated layer. Selective corrosion occurs in materials with different compositions and impurities. Under certain environmental conditions, some elements are corroded and leached out, leaving behind a spongy mass of uncorroded elements. Common examples include dezincification of brass, dealuminization of copper alloys, and graphitization of cast iron. Erosion-corrosion is a form of corrosion resulting from the alternating effects of fluid-induced wear and corrosion on metals. It is a common type of corrosion in valves, and it predominantly occurs on sealing surfaces. Prevention methods: Use materials that are resistant to corrosion and wear, improve the structural design, and employ cathodic protection, etc. Cavitation corrosion, also known as cavitation or air erosion, is a special form of wear corrosion. It is a bubble formed in a fluid; when it bursts, it generates a shock wave with pressures as high as 400 atmospheres, which destroys the metal protective layer and even tears apart the metal particles. Then it corrodes to form a film, and this process repeats continuously, causing the metal to corrode. Methods to prevent cavitation corrosion include using materials resistant to cavitation corrosion, machining surfaces with high smoothness, elastic protective layers, and cathodic protection. Fretting corrosion occurs when two contacting components are subjected to loads simultaneously, resulting in damage to the contact surface due to vibration and sliding. Moisture-vibration corrosion occurs at bolted joints, the connections between valve stems and closing elements, as well as between ball bearings and shafts. Protection can be achieved by applying lubricating grease to reduce friction, phosphating the surface, using cemented carbide, as well as improving surface hardness through spray welding or cold working. 　　Corrosion is damage caused by the diffusion of hydrogen atoms, generated in chemical reactions, into the interior of metals. Its forms include hydrogen blistering, hydrogen embrittlement, and hydrogen attack. High-strength steel and steel containing non-metals are prone to hydrogen blistering. Hydrogen bubbling easily occurs when oil contains sulfides and hydrides. Replacing porous boiling steel with hole-free calm steel, using rubber and plastic for protection, and adding corrosion inhibitors can prevent bubbling. In high-strength steel, the lattice height is highly deformed; upon the entry of hydrogen atoms, the lattice distortion increases, leading to fragility. Alloy steels containing nickel and lead should be selected; high-strength steels with high susceptibility to hydrogen embrittlement should be avoided. Hydrogen embrittlement should be prevented or minimized during welding, electroplating, and pickling. Under high temperature and pressure, hydrogen enters the metal and reacts chemically with certain elements, causing damage; this phenomenon is known as hydrogen embrittlement. Austenitic stainless steel is completely resistant to high-temperature hydrogen embrittlement. 3. Corrosion of non-metals Corrosion of non-metals is similar to that of metals. The vast majority of non-metallic materials are non-conductors, and therefore electrochemical corrosion does not occur in them; instead, corrosion results from purely chemical or physical processes. This is the main difference between non-metallic corrosion and metal corrosion. Non-metallic corrosion does not necessarily result in weight loss; instead, it often leads to an increase in weight. In the case of metallic corrosion, weight loss is the main phenomenon. Many cases of non-metallic corrosion are caused by physical factors, whereas physical factors play a very minor role in metallic corrosion. Internal corrosion is a common phenomenon in non-metals, while metallic corrosion mainly occurs on the surface. 　　When a metal material comes into contact with a medium, solutions or gases gradually diffuse into the interior of the material, causing a series of corrosion changes in the non-metallic material. Depending on the type and variety of the non-metallic material, the forms of corrosion vary. The forms of corrosion include dissolution, swelling, bubbling, softening, as well as decomposition, discoloration, deterioration, aging, hardening, and fracturing. However, from a comprehensive perspective, the corrosion resistance of non-metals is **significantly better than that of metal materials, while the strength and heat resistance of non-metals are lower than those of metal materials. II. Anti-corrosion measures for metal valves Electrochemical corrosion attacks metals in various forms; it occurs not only between two different metals, but also due to differences in the solubility of substances in the solution, differences in oxygen solubility, and minor variations in the internal structure of the metals, all of which can create potential differences that exacerbate corrosion. Some metals are not resistant to corrosion by themselves, but after corroding, they form an excellent protective layer, namely a passivation layer, which can prevent further corrosion by the surrounding medium. It can be seen that to achieve corrosion protection for metal valves, first, it is necessary to eliminate electrochemical corrosion; second, when electrochemical corrosion cannot be eliminated, a passivation film must be formed on the metal surface; third, non-metallic materials that do not suffer from electrochemical corrosion should be used as substitutes for metallic materials. Below are several anti-corrosion methods. 1. Select corrosion-resistant materials based on the medium
In the section “Selection of valves”, we introduced the media suitable for commonly used valve materials. However, this was merely a general introduction. In actual production, the corrosion caused by media is extremely complex. Even when the same valve material is used for the same medium, variations in the medium’s concentration, temperature, and pressure can result in different levels of corrosion on the material. For every 10°C increase in medium temperature, the corrosion rate increases by about 1 to 3 times. The concentration of the medium has a significant impact on the corrosion of valve materials; for example, lead experiences little corrosion in sulfuric acid with low concentrations, but corrosion increases sharply when the concentration exceeds 96%. In contrast, carbon steel suffers the most severe corrosion at a sulfuric acid concentration of around 50%; when the concentration increases to over 6%, corrosion decreases sharply. For example, aluminum is highly corrosive in concentrated nitric acid with a concentration of over 80%, but its corrosion is actually more severe in nitric acid of medium and low concentrations. Although stainless steel has strong corrosion resistance to dilute nitric acid, its corrosion worsens in nitric acid with a concentration of over 95%. 　　As can be seen from the above examples, the proper selection of valve materials should be based on specific circumstances; various factors affecting corrosion must be analyzed, and materials should be selected in accordance with relevant anti-corrosion manuals. 2. Use of non-metallic materials: Non-metals have excellent corrosion resistance. As long as the operating temperature and pressure of the valve meet the requirements of these materials, not only can corrosion problems be solved, but valuable metals can also be saved. The valve body, valve cover, lining, sealing surfaces, etc. of valves are commonly made of non-metallic materials, while gaskets and packing are mainly also made of non-metallic materials. The valve linings are made of plastics such as polytetrafluoroethylene and chlorinated polyethers, as well as rubbers such as natural rubber, neoprene, and nitrile rubber, while the valve body and the main part of the valve cover are made of ordinary cast iron or carbon steel. This ensures both the strength of the valve and its resistance to corrosion. Clamp valves are also designed based on the excellent corrosion resistance and remarkable deformability of rubber. These days, plastics such as nylon and polytetrafluoroethylene are increasingly used, along with natural rubber and synthetic rubber, to create various sealing surfaces and seals for use in different types of valves. These non-metallic materials used as sealing surfaces not only have excellent corrosion resistance but also good sealing properties, making them particularly suitable for use with media containing particles. Of course, their strength and heat resistance are relatively low, so their range of applications is limited. The emergence of flexible graphite has enabled non-metals to be used in high-temperature applications, solving the long-standing problem of leakage in fillers and gaskets; it is also an excellent high-temperature lubricant. 3. Spraying/painting coatings Coatings are the most widely used means of corrosion prevention; for valve products, they are an indispensable anti-corrosion material as well as a means of identification. Coatings also belong to non-metallic materials; they are typically made from synthetic resins, rubber pastes, vegetable oils, solvents, etc., and are applied to metal surfaces to isolate them from media and the atmosphere, thereby achieving anti-corrosion effects. Coatings are mainly used in environments with moderate corrosion, such as water, brine, seawater, and the atmosphere. The interior cavity of the valve is usually coated with anti-corrosion paint to prevent substances such as water and air from corroding the valve. Different colors are mixed into the paint to represent the materials used by Farn. Valves are painted every six months to one year. 4. Addition of corrosion inhibitors – Adding a small amount of certain special substances to the corrosive medium and corrosive agents can effectively slow down the rate of metal corrosion; such special substances are known as corrosion inhibitors. 　　The mechanism by which corrosion inhibitors control corrosion is that they promote the polarization of the cell. Corrosion inhibitors are mainly used in the medium and fillers. Adding corrosion inhibitors to the medium can reduce the corrosion of equipment and valves. For example, chromium-nickel stainless steel remains in an oxidized state over a wide range of concentrations in oxygen-free sulfuric acid, resulting in severe corrosion; however, the addition of small amounts of oxidizing agents such as copper sulfate or nitric acid can transform the stainless steel into a passive state, forming a protective layer on its surface that prevents erosion by the medium. In hydrochloric acid, the addition of small amounts of oxidizing agents can reduce the corrosion of titanium. Water is commonly used as the medium for pressure testing valves, but it can cause corrosion of the valves; adding a small amount of sodium nitrite to the water can prevent such corrosion. Asbestos fillers contain chlorides, which cause significant corrosion to valve stems. The chloride content can be reduced by using distilled water for washing, but this method is very difficult to implement and cannot be widely applied; esters are suitable for special cases. 　Non-metallic materials have excellent corrosion resistance; as long as the operating temperature and pressure of the valve meet the requirements of these materials, it not only solves the problem of corrosion but also helps to save precious metals. The valve body, valve cover, lining, sealing surfaces, etc. of valves are commonly made of non-metallic materials, while gaskets and packing are mainly also made of non-metallic materials. The valve linings are made of plastics such as polytetrafluoroethylene and chlorinated polyethers, as well as rubbers such as natural rubber, neoprene, and nitrile rubber, while the valve body and the main part of the valve cover are made of ordinary cast iron or carbon steel. This ensures both the strength of the valve and its resistance to corrosion. Clamp valves are also designed based on the excellent corrosion resistance and remarkable deformability of rubber. These days, plastics such as nylon and polytetrafluoroethylene are increasingly used, along with natural rubber and synthetic rubber, to create various sealing surfaces and seals for use in different types of valves. These non-metallic materials used as sealing surfaces not only have excellent corrosion resistance but also good sealing properties, making them particularly suitable for use with media containing particles. Of course, their strength and heat resistance are relatively low, so their range of applications is limited. The emergence of flexible graphite has enabled non-metals to be used in high-temperature applications, solving the long-standing problem of leakage in fillers and gaskets; it is also an excellent high-temperature lubricant. 3. Spraying/painting coatings Coatings are the most widely used means of corrosion prevention; for valve products, they are an indispensable anti-corrosion material as well as a means of identification. Coatings also belong to non-metallic materials; they are typically made from synthetic resins, rubber pastes, vegetable oils, solvents, etc., and are applied to metal surfaces to isolate them from media and the atmosphere, thereby achieving anti-corrosion effects. Coatings are mainly used in environments with moderate corrosion, such as water, brine, seawater, and the atmosphere. The interior cavity of the valve is usually coated with anti-corrosion paint to prevent substances such as water and air from corroding the valve. Different colors are mixed into the paint to represent the materials used by Farn. Valves are painted every six months to one year. 4. Addition of corrosion inhibitors – Adding a small amount of certain special substances to the corrosive medium and corrosive agents can effectively slow down the rate of metal corrosion; such special substances are known as corrosion inhibitors. 　　The mechanism by which corrosion inhibitors control corrosion is that they promote the polarization of the cell. Corrosion inhibitors are mainly used in the medium and fillers. Adding corrosion inhibitors to the medium can reduce the corrosion of equipment and valves. For example, chromium-nickel stainless steel remains in an oxidized state over a wide range of concentrations in oxygen-free sulfuric acid, resulting in severe corrosion; however, the addition of small amounts of oxidizing agents such as copper sulfate or nitric acid can transform the stainless steel into a passive state, forming a protective layer on its surface that prevents erosion by the medium. In hydrochloric acid, the addition of small amounts of oxidizing agents can reduce the corrosion of titanium. Water is commonly used as the medium for pressure testing valves, but it can cause corrosion of the valves; adding a small amount of sodium nitrite to the water can prevent such corrosion. Asbestos fillers contain chlorides, which cause significant corrosion to valve stems. The chloride content can be reduced by using distilled water for washing, but this method is very difficult to implement and cannot be widely applied; esters are suitable for special cases. The corrosion protection of valve stems is a matter of significant concern; extensive production experience has been accumulated, and surface treatment methods such as nitriding, boriding, chromium plating, and nickel plating are commonly used to enhance their corrosion resistance as well as their resistance to wear. Different surface treatments should be suitable for different valve stem materials and operating environments. For valve stems exposed to the atmosphere, water vapor, and asbestos packing, hard chromium plating or gas nitriding can be employed (ion nitriding is not recommended for stainless steel). In atmospheres containing hydrogen sulfide, electroplating with a high-phosphorus nickel coating provides good protective properties. 38CrMoAlA also exhibits corrosion resistance when subjected to ion or gas nitriding; however, hard chromium plating is not advisable for this material. After quenching and tempering, 2Cr13 becomes resistant to ammonia corrosion; carbon steel treated with gas nitriding also demonstrates resistance to ammonia corrosion. On the other hand, all phosphorus-nickel coatings lack resistance to ammonia corrosion. Gas-nitrided 38CrMoAlA possesses excellent corrosion resistance and overall performance characteristics, which is why it is frequently used in the manufacture of valve stems. 　　Small-diameter valve bodies and handwheels are also often chrome-plated to improve their corrosion resistance and to decorate the valves. 7. Thermal spraying Thermal spraying is a category of processes used for creating coatings, and it has become one of the new technologies for protecting material surfaces. It is a **key promotion project**. It is a surface strengthening process in which a high-energy-density heat source (such as gas combustion flames, arcs, plasma arcs, electric heating, gas explosions, etc.) is used to heat and melt metal or non-metallic materials, which are then sprayed in an atomized form onto a pre-treated base surface to form a spray coating; or the base surface is heated simultaneously, causing the coating to melt again on the substrate surface to form a spray-welded layer. Most metals and their alloys, metal oxide ceramics, cermets, as well as hard metal compounds can have coatings formed on metal or non-metal substrates using one or several thermal spraying methods. Thermal spraying can improve its surface properties such as corrosion resistance, wear resistance, and high-temperature resistance, thereby extending its service life. Thermally sprayed special functional coatings possess unique properties such as heat insulation, electrical insulation (or dielectricity), wear-resistant sealing, self-lubrication, thermal radiation, and electromagnetic shielding; thermally spraying can also be used to repair components. 8. Controlling the corrosive environment The term \"environment\" can be understood in two ways: in a broad sense, it refers to the environment surrounding the valve location as well as the medium flowing through it; in a narrow sense, it refers to the conditions around the valve installation site. Most environments are uncontrollable, and production processes cannot be changed arbitrarily. Controlled environmental methods, such as deoxygenation of boiler water and pH adjustment using caustic soda in oil refining processes, can be employed only when they do not cause any damage to the products or processes. From this perspective, the addition of corrosion inhibitors and electrochemical protection mentioned above also fall under controlling the corrosion environment. 　 The atmosphere is filled with dust, water vapor, and smoke; especially in production environments, toxic gases and fine particles emitted by brine solutions and equipment can cause varying degrees of corrosion to valves. Operators should clean and purge the valves regularly, as specified in the operating procedures, and top up the oil periodically; these are effective measures to control environmental corrosion. Installing protective covers on valve stems, placing underground valves in pits, and painting the valve surfaces are all methods of preventing corrosive substances from damaging the valves. Elevated ambient temperatures and air pollution can accelerate the corrosion of equipment and valves, especially in enclosed environments. Open-plan factories should be used as much as possible, or ventilation and cooling measures should be implemented to reduce environmental corrosion. 9. Improve processing techniques and valve structure designs. Corrosion protection for valves is a matter that needs to be considered from the design stage; it involves valve products with a rational structural design and proper manufacturing methods. It undoubtedly has a positive effect on reducing corrosion in valves. 　　Therefore, the design and manufacturing departments should improve those components with unreasonable structural designs, incorrect manufacturing methods, and a tendency to corrode, so as to meet the requirements under various operating conditions. 　The gaps at the valve connections provide an ideal environment for oxygen concentration cell corrosion. 　　Therefore, at the junction of the valve stem and the closing element, threaded connections should be avoided as much as possible; for welding valves, double-sided welding with continuous welding is required, as spot welding and lap welding can lead to corrosion. At the threaded connections of valves, PTFE tape and gaskets should be used. It not only provides good sealing but is also resistant to corrosion. Media that do not flow easily in dead corners can cause corrosion of valves. In addition to avoiding incorrect installation of the valves and making sure to remove any accumulated sediment, when manufacturing valve components, it is necessary to try to avoid structures that may lead to depressions, and valves should be equipped with drainage holes as much as possible. 　　Different metal contacts can form a point couple, accelerating the corrosion of the anodic metal; when selecting materials, it is necessary to avoid metal contacts with large potential differences that cannot form a passivation film. During the manufacturing and processing stages, stress corrosion occurs especially during welding and heat treatment; it is necessary to work on improving the processing methods, and appropriate protective measures such as annealing should be employed after welding. Improve the surface roughness of the valve stem processing surface as well as that of other valve components; the higher the surface roughness level, the greater the corrosion resistance. Improving the manufacturing processes and structures of fillers and gaskets, using flexible graphite and plastic fillers, as well as flexible graphite adhesive gaskets and PTFE-lined gaskets, can all enhance sealing performance and reduce corrosion of the gate valve stem and flange sealing surfaces.