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For valves operating in corrosive media, anti-corrosion is the most critical aspect of chemical equipment. If the metal materials of chemical valves cannot be selected correctly, a little carelessness may damage the equipment at least, or cause accidents or even disasters at worst. According to relevant statistics, about 60% of the damage to chemical equipment is caused by corrosion. Therefore, when selecting chemical equipment, we must first pay attention to the scientific nature of material selection. There is usually a misunderstanding that stainless steel is a "universal material" and that stainless steel is used regardless of the medium and environmental conditions. This is incorrect and very dangerous. Let’s talk about the key points of material selection for some commonly used chemical media.: 1. Sulfuric acid medium: As one of the highly corrosive media, sulfuric acid is an important industrial raw material with a wide range of uses. The corrosion of materials by sulfuric acid at different concentrations and temperatures is quite different. For concentrated sulfuric acid with a concentration above 80% and a temperature less than 80°C, carbon steel and cast iron have good corrosion resistance, but they are not suitable for high-speed flowing sulfuric acid and are not suitable for use as materials for pump valves. ； Ordinary stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) also have limited use 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 process) and high-alloy stainless steel (No. 20 alloy). Fluorine plastic has good sulfuric acid resistance, and using fluorine-lined pump valve (F46) is a more economical choice. If the pressure is too high and the temperature rises, the use point of the plastic valve will be affected, and you will have to choose a ceramic ball valve that is much more expensive. 2. Hydrochloric acid medium: Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), and molybdenum-containing high-silicon ferrosilicon can only be used in hydrochloric acid below 50°C and 30%. Contrary to metal materials, most non-metallic materials have good corrosion resistance to hydrochloric acid, so rubber-lined pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) are the best choices for transporting hydrochloric acid. But if the temperature of such a medium exceeds 150°C or the pressure is greater than 16 kg, any plastic (including polypropylene, fluoroplastics or even polytetrafluoroethylene) will not be able to handle it. There is no ideal valve on the market yet, but you can try the emerging ceramic ball valve. The advantages of this kind of valve are self-lubricating, low torque, no aging, and a much longer lifespan than ordinary valves. Its disadvantage is that the price is much higher than that of plastic valves. 3. Nitric acid medium: Most general metals are rapidly corroded and destroyed in nitric acid. Stainless steel is the most widely used nitric acid-resistant material and has good corrosion resistance to all concentrations of nitric acid at room temperature. It is worth mentioning that the corrosion resistance of molybdenum-containing stainless steel (such as 316, 316L) to nitric acid is not only no better than that of ordinary stainless steel (such as 304, 321), but sometimes even worse. For high-temperature nitric acid, titanium and titanium alloy materials are usually used. 4. Acetic acid medium: It is one of the most corrosive substances among organic acids. Ordinary steel will be severely corroded in acetic acid at all concentrations and temperatures. Stainless steel is an excellent acetic acid-resistant material. 316 stainless steel containing molybdenum is also suitable for high temperatures and dilute acetic acid vapor. For demanding requirements such as high-temperature, high-concentration acetic acid or other corrosive media, high-alloy stainless steel or fluoroplastic pumps can be selected. 5.Alkali (sodium hydroxide): Steel is widely used in sodium hydroxide solutions below 80°C and with a concentration of 30%. Many petrochemical plants still use ordinary steel at temperatures below 100°C and 75%. Although corrosion increases, the economy is good. The corrosion resistance of ordinary stainless steel to alkali liquid has no obvious advantage compared with cast iron. As long as a small amount of iron is allowed to be incorporated into the medium, stainless steel is not recommended. For high-temperature alkaline solutions, titanium and titanium alloys or high-alloy stainless steel are often used. 6. Ammonia (ammonia hydroxide): Most metals and non-metals corrode very slightly in liquid ammonia and ammonia (ammonia hydroxide). Only copper and copper alloys are not suitable for use. 7. Chlorine gas (liquid chlorine): The corrosion resistance of most metal valves to chlorine is very limited, especially when chlorine is mixed with water, including various alloy valves. In this case, PTFE valves are a very good choice, but chemical plants that produce chlor-alkali will find that: As the PTFE valve is used for a little longer and the torque increases, the problem of PTFE aging will become prominent. In this case, leakage will be fatal. You can consider replacing the original ordinary PTFE-lined valve with a PTFE-lined ceramic ball core. Taking advantage of the self-lubricating properties of ceramics and the corrosion resistance of PTFE will have perfect results. 8. Salt water (sea water): The corrosion rate of ordinary steel in sodium chloride solution, sea water, and salt water is not very high, and it generally needs to be protected by paint. ； Various types of stainless steel also have very low uniform corrosion rates, but may cause localized corrosion due to chloride ions. Generally, 316 stainless steel is better. 9. Alcohols, similar products, esters, ethers: Common alcohol media include methanol, ethanol, ethylene glycol, propanol, etc. Similar media include propylene and butylene, etc., ester media include various methyl esters, ethyl esters, etc., and ether media include methyl ether, ethyl ether, butyl ether, etc. They are basically non-corrosive and commonly used materials are applicable. The specific selection should also be based on the properties of the medium and related requirements. Reasonable selection should be made. It is also worth noting that homologs, esters, and ethers are soluble in a variety of rubbers, so avoid mistakes when selecting sealing materials. There are many other media that cannot be introduced one by one here. In short, you must not be casual and blind when selecting materials. You should consult more relevant information or learn from mature experience.