Thread Content
Metal valves can be considered key components in engineering equipment that are prone to corrosion and failure. Typically, materials such as those used for the sealing surfaces, valve stems, diaphragms, and small springs of metal valves are selected accordingly, while two or three different materials are used for the valve body and valve cover. For valves used with high-pressure, highly toxic, flammable, explosive, or radioactive media, materials with very low corrosivity are chosen. Under complex operating conditions such as those in the atmosphere or solutions, metal valves are not only subject to uniform corrosion of their metal surfaces at all times, but local forms of corrosion such as pitting, crevice corrosion, intergranular corrosion, delamination corrosion, stress corrosion, fatigue corrosion, selective corrosion, wear corrosion, cavitation corrosion, erosion-corrosion fatigue, and hydrogen corrosion also tend to occur in specific areas of the metal. Anti-corrosion measures for metal valves 01: Select corrosion-resistant materials based on the corrosive medium. In actual production, the corrosion caused by media is extremely complex. Even when using valve materials that are suitable for a particular medium, variations in the medium’s concentration, temperature, and pressure can result in different levels of corrosion on the materials. 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 severe corrosion at a sulfuric acid concentration of around 50%, while the corrosion rate drops sharply when the concentration increases above 6%. Just as aluminum is highly corrosive in nitric acid with a concentration of over 80%, it suffers more severe corrosion in nitric acid of moderate 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, by analyzing various factors that affect corrosion, and by choosing materials in accordance with relevant anti-corrosion manuals. 02 Use of non-metallic materials: Non-metals have excellent corrosion resistance. As long as the operating temperature and pressure of the valve are within the limits suitable for such materials, this not only helps to prevent corrosion but also allows for the savings of 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 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 deformation resistance 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 low, limiting their range of applications. The advent of flexible graphite has enabled non-metals to be used in high-temperature environments, solving the long-standing problem of leakage in fillers and gaskets; it is also an excellent high-temperature lubricant. 03 Metal surface treatment: Valve connection screws are commonly treated with galvanizing, chromium plating, or oxidation (blueing) to improve their resistance to atmospheric and chemical corrosion. In addition to the treatment methods mentioned above, other fasteners are also subjected to surface treatments such as phosphating, depending on the circumstances. For sealing surfaces and shut-off components with small diameters, surface treatments such as nitriding and boriding are often employed to improve their corrosion resistance and wear resistance. Valve discs made of 38CrMoAlA have a nitriding layer thickness ≥ 0. 4mm。 To prevent corrosion of valve stems, surface treatment methods such as nitriding, boriding, chromium plating, and nickel plating are widely used to enhance their corrosion resistance as well as their resistance to abrasion. Different surface treatment methods should be chosen according to the material of the valve stem and the operating environment. For valve stems exposed to atmospheric conditions, water vapor, and asbestos fillers, hard chromium plating or gas nitriding can be used (ion nitriding is not suitable for stainless steel). Valves operating in an environment with hydrogen sulfide benefit from high-phosphorus nickel plating, which provides good protection. 38CrMoAlA can also resist corrosion through ion nitriding or gas nitriding, but hard chromium plating is not appropriate for it. 2Cr13 can withstand ammonia corrosion after tempering; carbon steel treated by gas nitriding is also resistant to ammonia corrosion, whereas all phosphorus-nickel platings are not resistant to it. 38CrMoAlA materials treated by gas nitriding exhibit excellent corrosion resistance and overall performance, and they are often used to manufacture valve stems. Small-diameter valve bodies and handwheels are also often chrome-plated to improve their corrosion resistance and to decorate the valves. 04 Thermal Spraying: Thermal spraying is a process category used for coating creation, and it has become a new technology for protecting material surfaces. 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-function coatings possess unique properties such as thermal insulation, electrical insulation, wear-resistant sealing, self-lubrication, thermal radiation control, and electromagnetic shielding; thermally spraying can also be used to repair components. 05 Spraying paint: Paint is a widely used method for corrosion prevention, and it is an essential anti-corrosion material as well as an identification marker for valve products. 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 act as a barrier between the metal and the environment, thereby serving an anti-corrosion purpose. Coatings are mainly used in environments with relatively low corrosion, such as water, brine, seawater, and the atmosphere. The interior cavity of the valve is often coated with anti-corrosion paint to prevent corrosion by substances such as water and air. The paint contains different colors to indicate the materials used by Farn. Valves are painted every six months to once a year. 06 Add a corrosion inhibitor. The mechanism by which a corrosion inhibitor controls corrosion is that it promotes the chemical reaction in the battery. 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 convert the stainless steel to 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 presents many difficulties in practice and cannot be widely applied; esters are suitable for special cases. To protect the valve stem from corrosion by the asbestos packing, a corrosion inhibitor and a sacrificial metal are applied to the valve stem within the asbestos packing. The corrosion inhibitor, consisting of sodium nitrite and sodium chromate, helps to form a passivating film on the surface of the valve stem, thereby enhancing its corrosion resistance. The solvent facilitates the slow dissolution of the corrosion inhibitor while also serving as a lubricant. Zinc powder is added to asbestos as a sacrificial metal; in fact, zinc itself is also a type of corrosion inhibitor. It binds with the chlorides present in asbestos, thereby significantly reducing the chance of those chlorides coming into contact with the metal of the valve stem, thus achieving anti-corrosion effects. If corrosion inhibitors such as red lead and calcium lead sulfate are added to the coating, applying it to the surface of valves can prevent corrosion caused by the atmosphere. 07 Electrochemical protection: Electrochemical protection includes cathodic protection and anodic protection. When zinc is used to protect iron, zinc corrodes; zinc is known as a sacrificial metal. In production practice, anode protection is used less frequently, while cathode protection is applied more often. This cathodic protection method is used for large valves and critical valves; it is an economical, simple, and effective approach. Adding zinc to asbestos fillers to protect the valve stem also falls under cathodic protection methods. 08 Controlling the corrosive environment: The term \"environment\" can be understood in two ways – broadly, it refers to the environment surrounding the valve location as well as the medium flowing through it; narrowly, 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 alkalis 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 salts, vapors, 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, providing wells for floor valves, and painting the surface of valves are all methods to prevent 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. 09 Improving processing techniques and valve structure designs. Corrosion protection for valves is a consideration that starts from the design stage; it involves valve products with reasonable structural designs and appropriate processing methods. It undoubtedly has a positive effect on reducing corrosion in the control valves. Therefore, the design and manufacturing departments should improve those components with unreasonable structural designs, incorrect manufacturing methods, and a tendency to corrode, so that they can meet the requirements under various operating conditions. 10 For different types of corrosion affecting valve components, I have some solutions: To prevent intergranular corrosion in stainless steel valve parts, \"solution treatment followed by quenching\" can be employed, which involves heating to around 1100°C and then quenching in water. It is also advisable to use 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. Stress corrosion is the cracking that occurs under the simultaneous action of corrosion and tensile stress. Methods to prevent stress corrosion include eliminating or reducing stresses generated during welding and cold working through heat treatment, improving valve designs to avoid stress concentration, as well as employing electrochemical protection and applying anti-corrosion coatings. Measures such as adding corrosion inhibitors and applying compressive stress. Wear corrosion is a type of corrosion that results from the alternating action of fluid erosion and corrosion on metal; it is a common form of corrosion in valves, and it occurs most frequently on the sealing surfaces. Prevention methods: Use materials that are resistant to corrosion and wear, improve structural design, and employ cathodic protection, etc. 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 shut-off elements, as well as between ball bearings and shafts. Protection can be achieved by applying lubricating grease to reduce friction, surface phosphating, using cemented carbide, or increasing the surface hardness through shot peening or cold working. After welding, appropriate protective measures such as annealing should be employed to improve the surface roughness of the valve stem and that of other valve components; the higher the surface roughness, the stronger the corrosion resistance. Improving the processing techniques and structure of packings and gaskets—by using flexible graphite and plastic packings, as well as flexible graphite-coated gaskets and PTFE-wrapped gaskets—can enhance sealing performance and reduce corrosion on valve stems and flange sealing surfaces.