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In chemical industry, the medium transported in pipelines usually has certain corrosiveness, such as sulfuric acid, hydrochloric acid, nitric acid and other corrosive media. Corrosion protection is the most basic requirement of chemical equipment. Valves used for chemical industry are no exception, the selection of valves must be anti-corrosion. If the valve is selected incorrectly, it will damage the equipment, or even cause major accidents, and will cause injuries to the machinery and equipment, even to the human.
For different corrosive media, the selection of valves is also different. This paper will discusses the key points of valve material selection for some common chemical media:
most metal materials, including all kinds of stainless steel materials, are not resistant to hydrochloric acid corrosion. Molybdenum containing high silicon iron can only be used for hydrochloric acid below 50 ℃ and 30%. In contrast to metal materials, most non-metallic materials have good corrosion resistance to hydrochloric acid, so rubber lined valves (such as fluoroplastics) are the best choice for conveying hydrochloric acid. But if the temperature of the medium exceeds 150 ℃ or the pressure is greater than 16 kg, any plastic (including fluoroplastics and even PTFE) is also difficult to be competent. For hydrochloric acid exceeding this condition, there is no ideal valve on the market at present.
sulfuric acid, as one of the strong corrosive media, is an important industrial raw material with a wide range of uses. Sulfuric acid with different concentration and temperature has great difference in corrosion resistance of materials. For concentrated sulfuric acid with concentration above 80% and temperature below 80 ℃, carbon steel and cast iron have better corrosion resistance, but it is not suitable for sulfuric acid with high-speed flow so it is not suitable for valve materials. Stainless steel such as 304 and 316 also have limited use for sulfuric acid medium. Therefore, the valve for conveying sulfuric acid is usually made of high silicon cast iron (difficult to cast and process) and high alloy stainless steel (No. 20 alloy valve). Fluoroplastics have better sulfuric acid resistance. It is a better choice to use PTFE coated valves (recommended by Bundor valve products). If the pressure is too high and the temperature rises, the plastic valve will be impacted, so we can only choose the ceramic ball valve which is more expensive than it.
most of the general metals are destroyed by rapid corrosion 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 should be noted that the corrosion resistance of molybdenum containing stainless steel (such as 316, 316L) to nitric acid is not better or even worse than that of ordinary stainless steel (such as 304, 321).
acetic acid is one of the most corrosive substances in organic acids. Ordinary steel will be severely corroded in all concentrations and temperatures of acetic acid. Stainless steel is an excellent acetic acid resistant material, and 316 stainless steel with molybdenum can also be applied to high temperature and dilute acetic acid steam. For high temperature and high concentration acetic acid or containing other corrosive media, high alloy stainless steel or fluoroplastic pump can be selected.
the corrosion rate of ordinary steel in sodium chloride solution, sea water and salt water is not very high, so coating protection is generally required; all kinds of stainless steel also have very low uniform corrosion rate, but local corrosion may be caused by chloride ions, so 316 stainless steel is generally better.
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