【Haichuan Chemical Valve Management】Series Posts--What are the surface treatment processes for valves? What is its purpose?

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Proper management of valves is the foundation for stable operation. Everyone is welcome to engage in discussions and share insights on this series of posts: 【HaiChuan Chemical Valve Management】 series of posts: https://bbs.hcbbs.com/forum.php?mod=viewthread&tid=5719309 ---------------------------------------------------------------- Disclaimer: The content contained in this article is intended solely for technical exchange and reference purposes only; it does not constitute any form of professional engineering advice, design basis, or operational guidance. -----------------------------------------------------------------------------------Surface treatment of valves is a crucial factor in improving their performance and extending their service life. Its functions include corrosion prevention, wear resistance, improved sealing, and adaptation to specific operating conditions. Different treatment processes are designed for different purposes and can meet the needs of various industrial applications. Below, common treatment processes along with their functions are discussed in relation to actual operational conditions. The spraying process is the most widely used basic treatment method. Its principle involves applying a specific coating to the surface of the valve using high pressure or electrostatic forces, so that a dense protective film is formed after curing. Its function is to isolate the external environment from the valve body substrate, preventing the valve body from corroding due to exposure to air, moisture, or ordinary media. Additionally, it allows for adjustment of the applicable operating conditions based on the type of coating; it is mainly suitable for normal environments with no strong corrosivity or severe wear, offering both low cost and basic protection. This process is easy to implement and can be applied to valves of different sizes; it effectively extends the service life of valves in normal operating conditions and reduces the frequency of routine maintenance. It should be noted that conventional organic coatings have limited wear resistance and heat resistance, and are not suitable for high-temperature or highly abrasive environments. The electroplating process deposits a metal layer on the surface of the valve through electrolysis, creating a metal protective layer. Unlike spraying, the metal layer formed by electroplating bonds tightly to the valve body substrate and is not prone to falling off. Its primary function is to improve the wear resistance of the valve surface, and it is particularly suitable for the key moving parts of small-diameter valves, such as the valve core and valve stem – components where reducing friction and preventing sticking is necessary. The functions of different metal coatings vary. For instance, chromium plating is characterized by high hardness and excellent wear resistance; zinc plating primarily serves as a basic rust preventive measure; nickel plating, on the other hand, offers some resistance to mild corrosion. This process can effectively prevent seal failure caused by wear on the critical moving parts of valves. However, the coating is usually quite thin; it is prone to pitting corrosion in highly corrosive environments. Additionally, hydrogen embrittlement and environmental protection requirements must be taken into consideration. The lining process is a specialized treatment for highly corrosive operating conditions; it involves applying a thick layer of corrosion-resistant material with extremely high chemical stability inside the valve cavity and flow channels, thereby completely isolating the medium from the valve body substrate. Its primary function is to provide strong corrosion resistance, making it suitable for highly corrosive environments such as those with strong acids, strong bases, and strong organic solvents. Lining materials such as polytetrafluoroethylene and rubber possess good chemical stability, enabling them to resist the erosion of various highly corrosive substances. Meanwhile, some of these lining materials have a certain degree of elasticity, which allows them to compensate for minor deformations in the valve body and to withstand the scouring effect of media containing particles. This process can effectively prevent the valve body from being damaged due to severe corrosion, ensuring the long-term stable operation of the valve under harsh conditions; however, the thick lining layer has a certain impact on the dimensions of the valve’s flow channels. The surfacing process involves welding a layer of high-performance alloy material onto critical parts of valves; its main purpose is to enhance the wear resistance, temperature resistance, and corrosion resistance of the valve surface. It is primarily used in areas that are prone to wear and require high-level protection, such as the valve sealing surfaces. The deposited alloy layer forms a metallurgical bond with the substrate; this bond has high strength and sufficient thickness, enabling it to resist the effects of high temperatures, high pressures, and severe wear. As a result, leakage at the sealing surfaces due to wear or corrosion is prevented, thereby significantly extending the service life of the critical parts of the valve. This process is suitable for harsh environments with high temperatures and pressures, such as those in the petrochemical and power generation industries. However, it requires a large amount of heat input, which may cause deformation of the base material; therefore, appropriate post-weld treatment is necessary. Nitriding is a process in which nitrogen atoms are diffused into the surface of valve metal through chemical reactions to form a hardened layer. Its main purpose is to increase the hardness and wear resistance of the valve surface, while also enhancing its corrosion resistance to some extent. It is primarily used for precision components such as valve stems and balls that require continuous movement and are prone to wear. After this treatment, the surface hardness of the valve increases significantly, which helps to reduce frictional losses during operation and prevents the components from seizing or getting scratched. Moreover, the valve body experiences minimal deformation during the treatment process, allowing the precise dimensions of the parts to be maintained. It should be noted that conventional nitriding treatment may reduce the corrosion resistance of certain stainless steels; specialized low-temperature nitriding techniques can be used to avoid this issue in high-end applications. The electroless nickel plating process does not require an external current; it deposits a uniform nickel-phosphorus alloy layer on the surface of valves through a self-catalytic reaction. Its main function is to provide uniform corrosion protection and wear resistance, making it particularly suitable for the overall treatment of valves with complex shapes or those with small diameters. The coating formed by this process is uniform and dense, with no significant edge effects; it provides comprehensive protection for the valve surface against various mildly corrosive agents. Moreover, it possesses high hardness after appropriate heat treatment, which helps to reduce wear on the components. Chemical nickel plating meets the protection requirements of valves in various media such as the petroleum and chemical industries, as well as those with complex shapes; it is particularly suitable for internal cavity structures with irregular shapes where a uniform coating cannot be achieved through electroplating. Passivation is mainly used for stainless steel valves; it involves the removal of surface iron contamination through chemical methods, thereby promoting the formation of a dense chromium-rich oxide film on the surface. Its primary function is to enhance the pitting resistance of stainless steel valves, prevent rusting on their surfaces, and maintain the cleanliness of those surfaces. This process is suitable for industries with high hygiene standards such as food, pharmaceuticals, and drinking water; it prevents contamination of the fluid due to surface iron contamination or unstable oxide layers, thereby ensuring the safety and purity of fluid transport.