Selection of valves, often starting from the body material
Corrosion of valve bodies is primarily addressed through the proper selection of materials. Although there is a wealth of information on corrosion resistance, choosing the right material remains challenging due to the complexity of corrosion phenomena. For example, low-concentration sulfuric acid is highly corrosive to steel, while high-concentration sulfuric acid can actually form a passive layer on steel, offering protection. Similarly, hydrogen exhibits strong corrosiveness under high temperature and pressure, but its effect on steel is minimal in dry conditions. In contrast, when humidity is present, chlorine becomes highly corrosive, limiting the use of many materials.
Selecting the appropriate valve body material is not just about corrosion resistance—it also involves considering factors like pressure and temperature resistance, cost-effectiveness, and availability. This makes the decision process quite complex. One common solution is to apply lining measures, such as lead lining, aluminum lining, engineering plastic lining, or natural and synthetic rubber linings. These methods can be effective if the media conditions allow for it.
In cases where pressure and temperature are relatively low, using non-metallic materials for the valve body can be an effective way to control corrosion. Additionally, the outer surface of the valve body is also susceptible to atmospheric corrosion, so painting is a commonly used protective measure.
Valve corrosion typically refers to the damage that metal components experience in chemical or electrochemical environments. Since corrosion occurs naturally through the interaction between metal and its surroundings, the key to prevention is either isolating the metal from the environment or using more non-metallic, synthetic materials. The valve body, which includes the valve cover, makes up the majority of the valve’s weight and is constantly exposed to the medium. Therefore, material selection for the valve body is usually the starting point in the design process.
Valve body corrosion generally falls into two categories: chemical corrosion and electrochemical corrosion. The rate at which corrosion occurs depends on factors such as the medium's temperature, pressure, chemical properties, and the material's own corrosion resistance. Corrosion rates are typically categorized into six levels:
1. Excellent corrosion resistance: less than 0.001 mm/year
2. Good corrosion resistance: 0.001–0.01 mm/year
3. Moderate corrosion: 0.01–0.1 mm/year
4. Noticeable corrosion: 0.1–1.0 mm/year
5. Poor corrosion resistance: 1.0–10 mm/year
6. Severe corrosion: more than 10 mm/year
Understanding these levels helps engineers make informed decisions about material selection and maintenance strategies. It's essential to take a holistic approach, balancing performance, cost, and environmental conditions to ensure long-term reliability and safety.
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