Requirements for the material of valve components and their relationship with the medium

For valves operating in corrosive media, corrosion prevention is of paramount importance in chemical equipment. If the metal material used for such chemical valves is not selected properly, it can lead to equipment damage in mild cases, and in severe cases, it may cause accidents or even disasters. 1. Sulfuric acid medium: As one of the highly corrosive media, sulfuric acid is an important industrial material with a wide range of applications. Sulfuric acid at different concentrations and temperatures causes varying degrees of corrosion to materials. For concentrated sulfuric acid with a concentration of over 80% and a temperature below 80°C, carbon steel and cast iron exhibit good corrosion resistance; however, it is not suitable for sulfuric acid that flows at high speeds, and thus cannot be used as material for pumps and valves ； Common stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) also have limited utility in sulfuric acid media. Therefore, pump valves for transporting sulfuric acid are usually made of high-silicon cast iron (which is difficult to cast and machine) or highly alloyed stainless steel (grade 20 alloy). Fluoroplastics exhibit good resistance to sulfuric acid, making the use of fluorine-lined pumps and valves (F46) a more economical option. If the pressure is too high and the temperature rises, the performance of the plastic valve is affected, and in such cases one has to opt for a ceramic ball valve, which is much more expensive. 2. Hydrochloric acid medium: The vast majority of metal materials are not resistant to corrosion by hydrochloric acid (including various stainless steel materials); molybdenum-containing high-silicon iron can only be used in hydrochloric acid at 50°C and with a concentration of less than 30%. Unlike metal materials, the vast majority of non-metallic materials exhibit good corrosion resistance to hydrochloric acid; therefore, rubber-lined pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) are the best choices for transporting hydrochloric acid. However, if the temperature of such a medium exceeds 150°C or the pressure is greater than 16 kilograms, no plastic material (including polypropylene, fluoroplastics, and even polytetrafluoroethylene) will be suitable for use. At present, there are no ideal valves available on the market; but one could try the newly developed ceramic ball valves. The advantages of these valves are their self-lubricating properties, low torque requirement, resistance to aging, and a much longer service life compared to ordinary valves. Their drawback, however, is that their price is much higher than that of plastic valves. 3. Nitric acid medium: Most metals are rapidly corroded and damaged in nitric acid. Stainless steel is the most widely used material resistant to nitric acid; it exhibits good corrosion resistance against nitric acid at all concentrations at room temperature. It is worth noting that stainless steels containing molybdenum (such as 316 and 316L) do not have better corrosion resistance to nitric acid than ordinary stainless steels (such as 304 and 321), and in some cases their resistance is even lower. For high-temperature nitric acid, titanium and titanium alloy materials are typically used. 4. Acetic acid medium: It is one of the most corrosive organic acids; ordinary steel suffers severe corrosion in acetic acid at all concentrations and temperatures. Stainless steel is an excellent material resistant to acetic acid, and 316 stainless steel containing molybdenum can also be used in high-temperature environments as well as with dilute acetic acid vapors. For demanding applications such as high-temperature, high-concentration acetic acid or environments containing other corrosive agents, high-alloy stainless steel or fluoroplastic pumps can be used. 5. Bases (sodium hydroxide): Steel is widely used in sodium hydroxide solutions at temperatures below 80°C and with concentrations up to 30%. Many petrochemical plants also use ordinary steel at temperatures of 100°C and with concentrations below 75%; although corrosion increases, it remains cost-effective. Ordinary stainless steel does not have a significant advantage over cast iron in terms of corrosion resistance to alkaline solutions; it is not recommended to use stainless steel as long as a small amount of iron can be tolerated in the medium. For high-temperature alkaline solutions, titanium and titanium alloys or highly alloyed stainless steels are commonly used. 6. Ammonia (ammonium hydroxide): Most metals and non-metals suffer only mild corrosion in liquid ammonia and ammonium hydroxide solutions; only copper and its alloys are not suitable for use. 7. Chlorine (liquid chlorine): Most metal valves have very limited resistance to corrosion by chlorine, especially when chlorine is present in aqueous form. This applies to various alloy valves as well; in such cases, PTFE valves are a good choice. However, chemical plants that produce chlor-alkali compounds find that over time, the torque required to operate these PTFE valves increases, and the problem of PTFE aging becomes apparent. Leaks that occur under such conditions can be fatal. Replacing the conventional PTFE-lined valves with ones that use a ceramic core lined with PTFE could be considered; the self-lubricating properties of ceramics combined with the corrosion-resistant characteristics of PTFE would yield excellent results. 8. Saltwater (seawater): Ordinary steel experiences a low rate of corrosion in sodium chloride solutions as well as in seawater and brackish water; coating protection is generally required ； Various types of stainless steel also have very low uniform corrosion rates, but local corrosion may occur due to chloride ions; therefore, 316 stainless steel is generally the better choice. 9. Alcohols, ketones, esters, ethers: Common alcohol-based media include methanol, ethanol, ethylene glycol, propanol, etc.; ketone-based media include acetone, butanone, etc.; ester-based media include various methyl esters, ethyl esters, etc.; ether-based media include dimethyl ether, diethyl ether, butyl ether, etc. These substances generally have no corrosive properties, and most common materials can be used with them. When making a selection, it is necessary to consider the properties of the medium and relevant requirements in order to make a proper choice. It is also worth noting that alcohols, esters, and ethers are soluble in various rubbers, which helps to avoid mistakes when selecting sealing materials. There are many other media that cannot be introduced one by one here. In short, when selecting materials, one should not act casually or blindly; instead, it is necessary to consult relevant information and draw on proven experience.