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Corrosion has always been one of the most troublesome hazards of chemical equipment. If it is careless, it will damage the equipment, or even cause accidents or disasters. According to relevant statistics, about 60% of the damage of chemical equipment is caused by corrosion, so in the selection of chemical valves, we should first pay attention to the scientific nature of material selection. There is usually a misunderstanding that stainless steel is a "universal material", no matter what medium and environmental conditions are holding stainless steel valves, which is very dangerous.
For example, valves used in mining, water treatment plant or ship are easy to corroded, so the material choosing is very important. Otherwise it will cause big trouble.
The following are the main points of material selection for some commonly used chemical media:
As one of the strong corrosive media, sulfuric acid is an important industrial raw material with a 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. Not suitable for pump valves; ordinary stainless steel such as 304 (0Cr18Ni9), 316 (0Cr18Ni12Mo2Ti) is also limited in use for sulfuric acid media. Therefore, the pumps and valves that deliver sulfuric acid are usually made of high-silicon cast iron (difficult to cast and process) and high-alloy stainless steel (20# alloy). Fluoroplastics have better resistance to sulfuric acid, and fluorine-lined valves are a more economical choice.
Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), and molybdenum-containing high-silicon ferrosilicon 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 choices for delivering hydrochloric acid.
Generally, most metals are rapidly corroded and destroyed in nitric acid. Stainless steel is the most widely used nitric acid-resistant material and has good corrosion resistance to all concentrations of nitric acid at room temperature. It is worth mentioning that molybdenum-containing stainless steel (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), but sometimes even worse. For high-temperature nitric acid, titanium and titanium alloy materials are usually used.
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 acetic acid-resistant material. Molybdenum-containing 316 stainless steel is also suitable for high temperature and dilute acetic acid vapor. . For severe requirements such as high temperature and high concentration of acetic acid or other corrosive media, high alloy stainless steel valves or fluoroplastic valves can be selected.
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