Corrosion has always been one of the most headache hazards of chemical equipment. A little carelessness can damage equipment and cause accidents or even disasters. According to relevant statistics, about 60% of the destruction of chemical equipment is caused by corrosion, so the chemical selection of chemical pumps should first be paid attention to when selecting chemical pumps. There is usually a misconception that stainless steel is a "universal material". It is very dangerous to hold out stainless steel regardless of the medium and environmental conditions. Here are some key points for material selection for some commonly used chemical media:
1. Sulfuric acid As one of the strong corrosive media, sulfuric acid is an important industrial raw material with very wide applications. Sulfuric acid with different concentrations and temperatures has a greater difference in corrosion to 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. Not suitable for pump valve materials; ordinary stainless steels such as 304 (0Cr18Ni9), 316 (0Cr18Ni12Mo2Ti) are also of limited use for sulfuric acid media. Therefore, the pump valve for conveying sulfuric acid is usually made of high silicon cast iron (difficult to cast and process) and high alloy stainless steel (20 alloy). Fluorine plastic has better sulfuric acid resistance, and the use of fluorine-lined pump (F46) is a more economical option.
2. Hydrochloric acid Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), and molybdenum-containing high-silicon iron can only be used for hydrochloric acid at 50°C and below 30%. Contrary to metallic materials, most non-metallic materials have good corrosion resistance to hydrochloric acid, so lined rubber pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) are the best choice for the transportation of hydrochloric acid.
3. Most nitric acid general 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 to nitric acid is not only not better than ordinary stainless steel (such as 304, 321), sometimes even not as good. For high temperature nitric acid, titanium and titanium alloy materials are usually used.
4. Acetic acid is one of the most corrosive substances in organic acids. Ordinary steel will be severely corroded in acetic acid of all concentrations and temperatures. Stainless steel is an excellent acetic acid-resistant material. Molybdenum-containing 316 stainless steel is also suitable for high temperatures and Dilute acetic acid vapor. For high-temperature high-concentration acetic acid or containing other corrosive media and other harsh requirements, high alloy stainless steel or fluorine plastic pumps can be used.
5. Alkali (sodium hydroxide) steel is widely used in sodium hydroxide solution with a concentration of less than 80°C and a concentration of 30%. Many factories still use ordinary steel at a temperature of 100°C and a concentration of less than 75%. Although the corrosion increases, the economy is good . The corrosion resistance of ordinary stainless steel to alkaline solution has no obvious advantages compared with cast iron. As long as a small amount of iron is allowed to be mixed into the medium, stainless steel is not recommended. For high temperature lye, titanium and titanium alloys or high alloy stainless steels are mostly used.
6. Ammonia (ammonia hydroxide) Most metals and non-metals are slightly corroded in liquid ammonia and ammonia water (ammonia hydroxide), only copper and copper alloys are not suitable for use.
7. Salt water (sea water) The corrosion rate of ordinary steel in sodium chloride solution and sea water and salt water is not very high, generally need to be protected by paint; all kinds of stainless steel also have a very low uniform corrosion rate, but may be localized due to chloride ions Corrosion, usually 316 stainless steel is better.
8. Alcohols, ketones, esters, ethers, common alcohol mediums include methanol, ethanol, ethylene glycol, propanol, etc., ketone mediums include acetone, methyl ethyl ketone, etc., ester mediums have various methyl esters, Ethyl esters and other ether media include methyl ether, 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 properties 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 avoid errors when choosing sealing materials.
There are many other media that cannot be introduced one by one. In short, when selecting materials, do not be casual and blind. You should consult more relevant materials or learn from mature experience.
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