For valves under corrosive medium conditions, anti-corrosion is the most critical part of chemical equipment. If the metal materials of chemical valves cannot be selected correctly, a little carelessness can damage the equipment, or cause accidents or even disasters. According to relevant statistics, about 60% of the damage of chemical equipment is caused by corrosion. Therefore, the scientific nature of the material selection should be paid attention to when selecting the chemical equipment. There is usually a misunderstanding that stainless steel is a "universal material", regardless of the medium and environmental conditions, it is not correct and dangerous. Here are some key points of material selection for some commonly used chemical media:
1. Sulfuric acid medium , "As one of the strong corrosive media, sulfuric acid is an important industrial raw material with a very wide range of uses. Different concentrations and temperatures of sulfuric acid have great differences in the corrosion of materials. For concentrated sulfuric acid with a concentration of more than 80% and a temperature of less than 80°C, carbon steel and cast iron have good corrosion resistance, but it is not suitable for high-speed flowing sulfuric acid. It is not suitable for use as a material for pump valves; ordinary stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) have limited use for sulfuric acid media. Therefore, pump valves for transporting sulfuric acid are usually made of high-silicon cast iron (difficult to cast and process) and high-alloy stainless steel (alloy 20). Fluoroplastics have good resistance to sulfuric acid, and it is a more economical choice to use fluorine-lined pump valves (F46). If the pressure is too high and the temperature rises, the point of use of the plastic valve will be impacted, and you can only choose the more expensive ceramic ball valve.
2. Hydrochloric acid medium. Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), and high-silicon ferro-molybdenum can only be used in hydrochloric acid below 50°C and 30%. Contrary to metal materials, most non-metal materials have good corrosion resistance to hydrochloric acid, so rubber pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) are the best choice for transporting hydrochloric acid. However, if the temperature of such a medium exceeds 150°C, or the pressure is greater than 16 kg, any plastic (including polypropylene, fluoroplastics and even polytetrafluoroethylene) will not be competent, and there is no ideal valve on the market. , But you can try the emerging ceramic ball valve. The advantages of this valve are self-lubricating, low torque, no aging, and a much longer life than ordinary valves. Its disadvantage is that the price is much higher than that of plastic valves.
3. Nitric acid medium Most metals are rapidly corroded and destroyed in nitric acid. Stainless steel is the most widely used nitric acid resistant material. It has good corrosion resistance to all concentrations of nitric acid at room temperature. It is worth mentioning that stainless steel containing molybdenum ( Such as 316, 316L) the corrosion resistance of nitric acid is not better than ordinary stainless steel (such as 304, 321), sometimes even inferior. For high-temperature nitric acid, titanium and titanium alloy materials are usually used.
4. Acetic acid medium is one of the most corrosive substances in 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 and high concentration of acetic acid or other corrosive media, high alloy stainless steel or fluoroplastic pumps can be used.
5. Alkali (sodium hydroxide) steel is widely used in sodium hydroxide solution below 80°C and 30% concentration. There are also many petrochemical plants that still use ordinary steel when the temperature is below 100°C and 75%. Although corrosion increases, it is economical. good. The corrosion resistance of ordinary stainless steel to lye has no obvious advantages compared with cast iron. As long as a small amount of iron is allowed to be mixed in the medium, stainless steel is not recommended. For high-temperature lye, titanium and titanium alloy or high-alloy stainless steel are mostly used.
6. Ammonia (Ammonia Hydroxide) "Most metals and non-metals have slight corrosion in liquid ammonia and ammonia (Ammonia Hydroxide). Only copper and copper alloys are not suitable for use.
7. Chlorine (liquid chlorine) Most metal valves have limited resistance to chlorine corrosion, especially in the case of chlorine with water, including various alloy valves. In this case, PTFE valves are a very good choice. However, chemical plants that produce chlor-alkali will find that the PTFE valve takes a little longer and the torque increases, and the problem of PTFE aging will become prominent. The leakage that occurs in this case is fatal. It can be considered to replace the original ordinary PTFE-lined valve with a PTFE-lined ceramic ball core, and the self-lubricating property of ceramics and the corrosion resistance of PTFE will have a perfect effect.
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 generally must be protected by paint; all kinds of stainless steel also have a very low uniform corrosion rate, but it may be localized due to chloride ions Corrosion, usually 316 stainless steel is better.
9. Alcohols, ketones, esters and ethers. Common alcohol media include methanol, ethanol, ethylene glycol, propanol, etc., ketone media include acetone, butanone, etc., and ester media include various methyl esters, Ethyl and other ether media include methyl ether, ethyl ether, butyl ether, etc. They are basically non-corrosive, and commonly used materials can be applied. When selecting, a reasonable choice should be made according to the properties of the media and related requirements. It is also worth noting that ketones, esters, and ethers are soluble in a variety of rubbers, so errors should be avoided when selecting sealing materials.
There are many other media that cannot be introduced here. In short, you must not be arbitrarily and blindly when selecting materials. You should consult more relevant materials or learn from mature experience.