Valve material selection for different corrosive media
Corrosion, as one of the most troublesome maintenance materials in chemical equipment, will bring serious disasters to the chemical industry. Therefore, when selecting chemical valves, we must first pay attention to the scientific nature of material selection. There is usually a misunderstanding that stainless steel is a "universal material", so it is very dangerous to use stainless steel valves regardless of medium and environmental conditions.
The following are some common chemical media to talk about the main points of material selection:
Sulfuric acid: 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 a great difference in the corrosion of materials. For concentrated sulfuric acid with a concentration above 80% and a temperature below 80°C, carbon steel and cast iron have better corrosion resistance, but it is not suitable for high-speed flowing sulfuric acid. It is not suitable as a material for pump valves; ordinary stainless steel such as 304 (0Cr18Ni9), 316 (0Cr18Ni12Mo2Ti) has limited use for sulfuric acid media. Therefore, pumps and valves for transporting sulfuric acid are usually made of high-silicon cast iron (difficult to cast and process) and high-alloy stainless steel (alloy No. 20). Fluoroplastics have better resistance to sulfuric acid, and it is a more economical choice to use fluorine-lined valves.
Acetic acid: It 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 material for acetic acid resistance. Molybdenum-containing 316 stainless steel is also suitable for high temperature and dilute Acetic acid vapour. For high-temperature, high-concentration acetic acid or other corrosive media, high-alloy stainless steel valves or fluoroplastic valves can be used.
Hydrochloric acid: Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), and high silicon ferrosilicon containing molybdenum can only be used for 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 valves and plastic valves (such as polypropylene, fluoroplastics, etc.) are the best choice for conveying hydrochloric acid.
Nitric acid: Most of the 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 that of ordinary stainless steel (such as 304, 321), sometimes even worse. For high temperature nitric acid, titanium and titanium alloy materials are usually used.