Top tricks to combat valve corrosion

　　Metal valves can be considered key components in engineering equipment that are *easily* prone to corrosion and failure. Typically, the sealing surfaces, valve stems, diaphragms, and small springs of metal valves generally use first-grade materials, while the valve body and valve cover use second- or third-grade materials. For valves used in high-pressure, highly toxic, flammable, explosive, or radioactive media, materials with very low corrosiveness are selected.

　　Valve Corrosion

　　Under complex working conditions such as atmospheric exposure or immersion in solutions, metal valves not only undergo uniform corrosion on the metal surface, but are also extremely prone to pitting corrosion, crevice corrosion, intergranular corrosion, exfoliation corrosion, stress corrosion, fatigue corrosion, selective corrosion, wear corrosion, cavitation corrosion, fretting corrosion, hydrogen embrittlement, and other forms of localized corrosion at specific locations on the metal.

　　Corrosion Prevention Measures for Metal Valves

　　1. Select corrosion-resistant materials based on the corrosive medium

　　In actual production, the corrosion of media is very complex. Even if the valve materials used in a single medium are the same, the concentration, temperature, and pressure of the medium will affect the corrosion of the material differently. For every 10℃ increase in medium temperature, the corrosion rate increases by approximately 1 to 3 times. The concentration of the medium has a significant impact on the corrosion of valve materials. For example, lead experiences minimal corrosion in low-concentration sulfuric acid, but corrosion increases sharply when the concentration exceeds 96%. Conversely, carbon steel experiences the most severe corrosion when the sulfuric acid concentration is around 50%, but corrosion decreases sharply when the concentration increases above 6%. Similarly, aluminum exhibits strong corrosiveness in concentrated nitric acid with a concentration above 80%, but corrosion is more severe in medium and low concentrations of nitric acid. Although stainless steel exhibits strong corrosion resistance to dilute nitric acid, corrosion increases in concentrated nitric acid above 95%. From these examples, it can be seen that the correct selection of valve materials should be based on specific circumstances, analyzing various factors affecting corrosion, and selecting materials according to relevant corrosion prevention manuals.

　　2. Use non-metallic materials

　　Non-metallic materials have excellent corrosion resistance. As long as the operating temperature and pressure of the valve meet the requirements of the non-metallic material, it can not only solve the corrosion problem but also save precious metals. The valve body, valve cover, lining, and sealing surfaces are often made of non-metallic materials, while gaskets and packing are mainly made of non-metallic materials. Using plastics such as polytetrafluoroethylene and chlorinated polyether, as well as natural rubber, chloroprene rubber, and nitrile rubber, as valve linings, while the valve body and valve cover are made of cast iron or carbon steel, ensures both valve strength and corrosion resistance. The pinch valve is also designed based on the excellent corrosion resistance and deformation properties of rubber. There is an increasing use of nylon, polytetrafluoroethylene, and other plastics, as well as natural and synthetic rubber, to make various sealing surfaces and seals for various valves. These non-metallic materials used as sealing surfaces not only have good corrosion resistance but also good sealing performance, especially suitable for use with media containing particles. Of course, their strength and heat resistance are relatively low, limiting their range of application. The emergence of flexible graphite has brought non-metallic materials into the high-temperature field, solving the difficult problem of leakage from packing and gaskets, and also serving as an excellent high-temperature lubricant.

　　3. Metal surface treatment

　　Valve connections: Valve connection screws are commonly treated with galvanizing, chroming, or oxidation (bluing) to improve their resistance to atmospheric and medium corrosion. In addition to the above methods, other fasteners are also treated with phosphating, depending on the situation.

　　Sealing surfaces and closing parts with small diameters are often treated with processes such as nitriding and boronizing to improve their corrosion resistance and wear resistance. For valve flaps made of 38CrMoAlA, the nitrided layer is ≥0.4mm.

　　Valve stem corrosion prevention widely uses surface treatment processes such as nitriding, boronizing, chromium plating, and nickel plating to improve its corrosion resistance and wear resistance. Different surface treatments should be suitable for different valve stem materials and working environments. For valve stems in contact with atmospheric, steam media, and asbestos packing, hard chromium plating and gas nitriding processes can be used (stainless steel should not use ion nitriding); valves in a hydrogen sulfide atmosphere can use electroplated high-phosphorus nickel plating for better protection; 38CrMoAlA can also be corrosion-resistant using ion and gas nitriding, but hard chromium plating is not suitable; 20Cr13, after tempering, can resist ammonia corrosion, and gas-nitrided carbon steel can also resist ammonia corrosion, while all phosphorus-nickel plating is not resistant to ammonia corrosion; gas-nitrided 38CrMoAlA material has excellent corrosion resistance and comprehensive performance, and is often used to make valve stems.

　　Small-diameter valve bodies and handwheels are also often chrome-plated to improve their corrosion resistance and enhance the valve's appearance.

　　4. Thermal spraying

　　Thermal spraying is a type of coating preparation process and has become one of the new technologies for surface protection of materials. It uses a high-energy-density heat source (gas combustion flame, arc, plasma arc, electric heating, gas explosion, etc.) to heat and melt metal or non-metal materials, and then spray them in an atomized form onto a pre-treated substrate surface to form a sprayed coating, or simultaneously heat the substrate surface, allowing the coating to remelt on the substrate surface to form a spray-welded layer for surface strengthening. Most metals and their alloys, metal oxide ceramics, metal-ceramic composites, and hard metal compounds can be used with one or more thermal spraying methods to form coatings on metal or non-metal substrates.

　　Thermal spraying can improve surface corrosion resistance, wear resistance, and high-temperature resistance, extending service life. Thermal spraying special functional coatings have special properties such as heat insulation, insulation (or anti-electric), self-lubricating, thermal radiation, and electromagnetic shielding; thermal spraying can also be used to repair components.

　　5. Spray coating

　　Coatings are widely used as a corrosion prevention method, and are also a corrosion-resistant material and identification mark on valve products. Coatings are also non-metallic materials, usually made from synthetic resins, rubber pastes, vegetable oils, solvents, etc., and are coated on the metal surface to isolate the medium and atmosphere, achieving corrosion prevention. Coatings are mainly used in environments with less severe corrosion, such as water, salt water, seawater, and atmosphere. The valve cavity is usually coated with anti-corrosion paint to prevent corrosion of the valve by water, air, and other media. Different colors are added to the paint to indicate the materials used. Valves are usually sprayed with coatings once every six months to a year.

　　6. Add corrosion inhibitors

　　The mechanism by which corrosion inhibitors control corrosion is that they promote battery polarization. Corrosion inhibitors are mainly used in the media and filler. Adding corrosion inhibitors to the media can reduce the corrosion of equipment and valves. For example, chromium-nickel stainless steel is in a passivated state in oxygen-free sulfuric acid within a large solubility range, and the corrosion is severe. However, adding a small amount of oxidizing agents such as copper sulfate or nitric acid can make the stainless steel turn passive, forming a protective film on the surface to prevent the medium from erosion. In hydrochloric acid, if a small amount of oxidizing agent is added, the corrosion of titanium can be reduced. Water is often used as the test medium for valve pressure testing, which can easily cause valve corrosion. Adding a small amount of sodium nitrite to the water can prevent water from corroding the valve. Asbestos packing contains chlorides, which greatly corrodes the valve stem. Although the washing method with distilled water can reduce the chloride content, this method is difficult to implement and cannot be widely promoted, except for special needs.

　　In order to protect the valve stem and prevent corrosion of the asbestos packing, corrosion inhibitors and sacrificial metals are applied to the asbestos packing and the valve stem. Corrosion inhibitors such as sodium nitrite and sodium chromate can form a passivation film on the surface of the valve stem, improving the corrosion resistance of the valve stem; solvents can slowly dissolve the corrosion inhibitor and also have a lubricating effect; adding zinc powder as a sacrificial metal to the asbestos. In fact, zinc is also a corrosion inhibitor, which can first combine with the chlorides in the asbestos, greatly reducing the chance of chlorides contacting the valve stem metal, thus achieving corrosion prevention. If red lead, lead calcium acid, etc. are added to the coating as corrosion inhibitors, spraying them on the valve surface can prevent atmospheric corrosion.

　　7. Electrochemical protection

　　Electrochemical protection includes anodic protection and cathodic protection. For example, when zinc protects iron, zinc is corroded, and zinc is called a sacrificial metal. In production practice, anodic protection is less used, while cathodic protection is more widely used. This cathodic protection method is used for large and important valves, and it is an economical, simple, and effective method. Adding zinc to asbestos packing to protect the valve stem also belongs to the cathodic protection method. 8. Control the corrosion environment

　　The so-called environment has both broad and narrow senses. The broad sense refers to the environment around the valve installation and the medium flowing inside it; the narrow sense refers to the conditions around the valve installation. Most environments cannot be controlled, and the production process cannot be arbitrarily changed. Only when it will not cause damage to the product, process, etc., can the method of controlling the environment be used, such as deoxygenation of boiler water, and adjusting the PH value with alkali in the oil refining process. From this point of view, the above-mentioned addition of corrosion inhibitors and electrochemical protection also belong to controlling the corrosion environment.

　　The atmosphere is full of dust, water vapor, and smoke. Especially in the production environment, such as smoke, halogen, toxic gases and fine powders emitted by equipment, will cause different degrees of corrosion to the valve. Operators should regularly clean and blow the valve, and regularly add oil according to the regulations in the operating procedures. This is an effective measure to control environmental corrosion. Installing protective covers on the valve stem, setting ground pits for ground valves, and spraying paint on the valve surface are all methods to prevent corrosive substances from corroding the valve. Increased environmental temperature and air pollution, especially for equipment and valves in closed environments, will accelerate their corrosion. Open-type factories should be used as much as possible, or ventilation and cooling measures should be adopted to reduce environmental corrosion.

　　9. Improve processing technology and valve structure

　　Corrosion protection of valves is a problem that should be considered from the design stage. A valve product with a reasonable structural design and correct process methods will undoubtedly have a good effect on reducing valve corrosion.

　　Therefore, the design and manufacturing departments should improve those components with unreasonable structural design, incorrect process methods, and easy corrosion, so that they can meet the requirements of various different operating conditions. For different types of valve component corrosion, the editor has a trick: Methods to prevent intergranular corrosion in austenitic stainless steel valve parts include: "solution quenching" treatment, that is, heating to about 1100 ℃ and water quenching, selecting austenitic stainless steel containing titanium and niobium, and with a carbon content of less than 0.03%, to reduce the generation of chromium carbide.

　　Stress corrosion occurs when cracking occurs under the simultaneous action of corrosion and tensile stress. Methods to prevent stress corrosion include: eliminating or reducing stress generated during welding and cold processing through heat treatment, improving unreasonable valve structures to avoid stress concentration, using electrochemical protection, spraying anti-corrosion coatings, adding corrosion inhibitors, and applying compressive stress.

　　Erosion corrosion is a corrosion form caused by the alternating action of fluid erosion and corrosion on metal, which is a common type of corrosion in valves, and this corrosion often occurs on sealing surfaces. Prevention methods: select corrosion-resistant and wear-resistant materials, improve structural design, and use cathodic protection.

　　Fretting corrosion is the damage caused by vibration and sliding on the contact surface due to the simultaneous load on two contacting parts. Fretting corrosion occurs in bolt connections, valve stem and closing member connections, ball bearings and shafts, etc. Lubricating grease can be used to reduce friction, surface phosphating, the use of cemented carbide, and the use of spraying or cold working to improve surface hardness for protection. After welding, corresponding protective measures such as annealing should be used as much as possible. Improve the surface roughness of the valve stem and other valve parts. The higher the surface roughness level, the stronger the corrosion resistance. Improve the processing technology and structure of packing and gaskets, use flexible graphite and plastic packing, and flexible graphite bonded gaskets and PTFE-wrapped gaskets can improve sealing performance and reduce corrosion of the valve stem and flange sealing surfaces.