How to choose corrosion-resistant valves

For valves under corrosive media conditions, corrosion prevention is the most important aspect of chemical equipment. If the metal material of chemical valves is not selected correctly, a slight carelessness may damage the equipment, or cause accidents or even disasters. According to relevant calculations, about 60% of damage to chemical equipment is caused by corrosion. Therefore, the scientific nature of material selection must first pay attention to the selection of chemical equipment. There is a general misunderstanding that stainless steel is an "all-purpose material" regardless of medium and environmental conditions, which is incorrect and dangerous. Let’s first talk about the keys to selecting some commonly used chemical media:

1. Sulfuric acid medium: As one of the highly corrosive media, sulfuric acid is an important industrial raw material with a wide range of uses. Sulfuric acid at different concentrations and temperatures has different corrosive effects on materials. For concentrated sulfuric acid with a concentration above 80% and a temperature below 80°C, carbon steel and cast iron have good corrosion resistance, but are not suitable for high-speed flowing sulfuric acid. , not suitable for pump and valve materials; 304 (0Cr18Ni9), 316 (0Cr18Ni12Mo2Ti) and other general stainless steels have limited use in sulfuric acid media. Therefore, pump valves for conveying sulfuric acid generally use high-silicon cast iron (difficult to forge) and high-alloy stainless steel (Alloy 20). Fluorine plastic has good sulfuric acid resistance.

2. Hydrochloric acid medium: Most metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials). High silicon ferromolybdenum can only be used in 30% hydrochloric acid below 50°C. Contrary to metallic materials, most non-metallic materials have outstanding corrosion resistance to hydrochloric acid, so rubber pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) are the best choices for transporting hydrochloric acid.

3. Nitric acid medium: Most metals are rapidly corroded and destroyed in nitric acid. Stainless steel is the most widely used nitric acid-resistant material. It has excellent corrosion resistance to all concentrations of nitric acid at room temperature. It is worth mentioning that the nitric acid corrosion resistance of molybdenum-containing stainless steel (such as 316, 316L) is not only 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 generally used.

4. Acetic acid medium: It is one of the most corrosive substances among organic acids. In general, steel is severely corroded by acetic acid at all concentrations and temperatures. Stainless steel is an excellent material that is resistant to acetic acid. 316 stainless steel containing molybdenum is also suitable for high temperatures and dilute acetic acid vapor. For demanding requirements such as high temperature, high concentration of acetic acid or other corrosive media, high-alloy stainless steel or fluoroplastic pumps can be selected.

5. Alkali (sodium hydroxide): Steel is widely used in sodium hydroxide solutions below 80°C and 30% concentration. There are also many petrochemical plants that still use ordinary steel at temperatures below 100°C and 75% below. Although it is corrosive and additive, it is economical. Good sex. Generally speaking, the corrosion resistance of stainless steel to alkali solutions has no obvious advantage compared with cast iron. Just allow a small amount of iron to be mixed into the medium. Stainless steel is not recommended. For high-temperature alkaline solutions, titanium and titanium alloys or high-alloy stainless steel are often used. 6. Ammonia (Ammonia Hydroxide): Most metals and non-metals have extremely slight corrosion in liquid ammonia and ammonia (Ammonia Hydroxide), as long as copper and copper alloys are not suitable for use. 7. Salt water (sea water): Generally, the corrosion rate of steel in sodium chloride solution, sea water and salt water is not very high, and generally requires paint maintenance; various stainless steels also have extremely low uniform corrosion rates, but they may be subject to localized corrosion due to chloride ions. , Generally 316 stainless steel is better. 8. Alcohols, ketones, esters, and ethers: The most common alcohol media include methanol, ethanol, ethylene glycol, propanol, etc., ketone media include acetone, methyl ethyl ketone, etc., and ester media include various methanols. . Esters, ethyl esters, etc., and ether media include methyl ether, ethyl ether, butyl ether, etc., which are basically non-corrosive and can be used with commonly used materials. The specific selection should also be made reasonably based on the characteristics and relevant requirements of the media. It is also worth noting that ketones, esters, and ethers are soluble in a variety of rubbers, preventing errors when selecting sealing materials. There are many other media that cannot be introduced here. In short, you must not be casual and blind when choosing materials. You should consult more relevant information or learn from mature experience.