Material selection of chemical valves
Release date:
2019-02-19
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Abstract
Corrosion has always been one of the most common hazards of chemical equipment. If you are careless, the equipment will be damaged lightly, and the heavy will cause accidents and even disasters.According to relevant statistics, about 60% of the damage to chemical equipment is caused by corrosion. Therefore, when selecting chemical valves, we must first pay attention to the scientific nature of the material selection.There is usually a misunderstanding that stainless steel is a “universal material”, no matter what medium and environmental conditions can produce stainless steel valves, which is very dangerous.
Let's talk about the main points of material selection for some commonly used chemical media：
Sulfuric acid
As one of the highly corrosive media, sulfuric acid is an important industrial raw material with a wide range of uses.Sulfuric acid of different concentrations and temperatures has a large difference in the corrosion of materials. For concentrated sulfuric acid with a concentration of more than 80% and a temperature of less than 80℃, carbon steel and cast iron have good corrosion resistance, but it is not suitable for high-speed flow of sulfuric acid and is not suitable for use as a material for pumps and valves; ordinary stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) are also of limited use for sulfuric acid media.Therefore, the pump valves for conveying sulfuric acid are usually made of high-silicon cast iron (difficult to cast and process) and high-alloy stainless steel (No. 20 alloy).Fluoroplastic has good sulfuric acid resistance, and the use of fluorine-lined valves is a more economical choice.
Hydrochloric acid
Although most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), molybdenum-containing high ferrosilicon 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 and plastic valves (such as polypropylene, fluoroplastics, etc.) are the best choices for conveying hydrochloric acid.
Nitric acid
Most general metals are quickly 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 molybdenum-containing stainless steel (such as 316, 316L) has corrosion resistance to nitric acid. Not only is it not better than ordinary stainless steel (such as 304, 321), sometimes it is even inferior.For high-temperature nitric acid, titanium and titanium alloy materials are usually used.
Acetic acid
It is one of the most corrosive substances in organic acids. Ordinary steel will be severely corroded in acetic acid at all concentrations and temperatures. Stainless steel is an excellent material resistant to acetic acid. Molybdenum-containing 316 stainless steel can also be applied to high temperature and dilute acetic acid steam.For demanding requirements such as high temperature and high concentration of acetic acid or other corrosive media, high-alloy stainless steel valves or fluorine plastic valves can be selected.
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