For valves under corrosive medium conditions, corrosion […]
For valves under corrosive medium conditions, corrosion prevention is the most important part of chemical equipment. If the metal materials of chemical valves cannot be selected correctly, a little carelessness will damage the equipment, or cause accidents and even disasters. According to relevant calculations, about 60% of the damage of chemical equipment is caused by corrosion. Therefore, the scientific nature of material selection should be the first and foremost attention to the selection of chemical equipment. Generally, there is a misunderstanding that stainless steel is an "all-round material", regardless of the medium and environmental conditions, it is not correct and dangerous. Let’s talk about the key to selecting materials 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 wide range of uses. Sulfuric acid with different concentrations and temperatures has different corrosion effects on materials. Regarding concentrated sulfuric acid with a concentration above 80% and a temperature less than 80℃, 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 the material of pump valve; general stainless steel such as 304 (0Cr18Ni9), 316 (0Cr18Ni12Mo2Ti) has limited use for sulfuric acid medium. Therefore, pump valves for transporting sulfuric acid are generally made of high-silicon cast iron (difficult forging and processing) and high-alloy stainless steel (alloy 20). Fluoroplastics have better sulfuric acid resistance, and it is a more economical choice to use fluorine-lined pump valves (F46).
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 outstanding corrosion resistance to hydrochloric acid, so rubber pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) are the best choice for transporting hydrochloric acid.
3. Nitric acid medium: most metals are rapidly corroded and damaged in nitric acid. Stainless steel is the most widely used nitric acid resistant material. It has outstanding corrosion resistance to all concentrations of nitric acid at room temperature. It is worth mentioning that stainless steel containing molybdenum The corrosion resistance of nitric acid (such as 316, 316L) is not only better than that of ordinary stainless steel (such as 304, 321), and sometimes even worse. For high-temperature nitric acid, titanium and titanium alloy materials are generally used.
4. Acetic acid medium: It is one of the most corrosive substances in organic acids. Generally, 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 temperature and dilute acetic acid vapor. High-alloy stainless steel or fluorine plastic pumps can be selected for severe requirements such as high temperature and high concentration of acetic acid or rich in other corrosive media.
5. Alkali (sodium hydroxide): steel is widely used in sodium hydroxide solution below 80℃ and 30% concentration. There are also many petrochemical plants that still use general steel when it is below 100℃ and 75%. Although it is corrosive and added, it is economical. Good sex. Generally, the corrosion resistance of stainless steel to lye has no obvious advantages compared with cast iron. It only needs to allow a small amount of iron to be mixed in the medium. Stainless steel is not recommended. With regard to 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 very slight corrosion in liquid ammonia and ammonia (Ammonia Hydroxide), as long as copper and copper alloys are not suitable for use. 7. Salt water (sea water): The corrosion rate of general steel in sodium chloride solution, sea water and salt water is not very high, and paint maintenance is generally required; all kinds of stainless steel also have a very low uniform corrosion rate, but it may be localized due to chloride ions For sexual corrosion, 316 stainless steel is generally better. 8. Alcohols, ketones, esters, ethers: The most common alcohol media include methanol, ethanol, ethylene glycol, propanol, etc., ketone media include acetone, butanone, etc., and ester media include various methyl alcohols. Esters, ethyl esters, etc., ether media include methyl ether, ethyl ether, butyl ether, etc. They are basically non-corrosive, and commonly used materials can be applied. The specific selection should also be based on the characteristics of the media and related requirements to make a reasonable choice. It is also worth noting that ketones, esters, and ethers are soluble in a variety of rubbers to prevent mistakes when selecting sealing materials. There are many other media that cannot be introduced here one by one. In short, you must not be arbitrarily and blindly when selecting materials. You should consult more relevant materials or learn mature experience.