Principles For Selecting Valves In Corrosive Media

In the Lined Valve under the working conditions of corrosive media, corrosion prevention is the most critical part of chemical equipment. If the metal material of the chemical valve is not selected correctly, the equipment may be damaged at the least, and accidents or even disasters may occur at the worst. According to relevant statistics, about 60% of the damage to chemical equipment is caused by corrosion. Therefore, when selecting chemical equipment, we must first pay attention to the scientific selection of materials. There is usually a misunderstanding that stainless steel is a "universal material" and stainless steel is used regardless of the medium and environmental conditions. This is incorrect and very dangerous. The following is a discussion of the key points of material selection for some commonly used chemical media

Hydrochloric acid

The vast majority of metal materials are not resistant to hydrochloric acid corrosion (including various stainless steel materials), and 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 lined rubber pumps and plastic pumps (such as polypropylene, fluoroplastics, etc.) are the best choices for transporting hydrochloric acid

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 of different concentrations and temperatures has a great difference 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 they are not suitable for high-speed flowing sulfuric acid and are not suitable for use as pump and valve materials; ordinary stainless steel such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) are also limited in use for sulfuric acid media. Therefore, pumps and valves for conveying sulfuric acid are usually made of high-silicon cast iron (difficult to cast and process) and high-alloy stainless steel (Alloy 20). Fluoroplastics have good resistance to sulfuric acid, and the use of fluorine-lined pumps and valves (F46) is a more economical choice

Nitric Acid

Most general metals are quickly corroded and destroyed in nitric acid. Stainless steel is the most widely used nitric acid-resistant material and has good corrosion resistance to nitric acid of all concentrations at room temperature. It is worth mentioning that the corrosion resistance of molybdenum-containing stainless steel (such as 316, 316L) 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

Acetic acid

It is one of the most corrosive substances among 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 can also be used for high temperature and dilute acetic acid vapor. For demanding requirements such as high-temperature and high-concentration acetic acid or other corrosive media, high-alloy stainless steel or fluoroplastic pumps can be selected

Alkali (sodium hydroxide)

Steel is widely used in sodium hydroxide solutions below 80°C and within 30% concentration. There are also many petrochemical plants that still use ordinary steel at 100°C and below 75%. Although corrosion increases, it is economical. Ordinary stainless steel has no obvious advantage over cast iron in corrosion resistance to alkali solution. As long as a small amount of iron is allowed to be added to the medium, stainless steel is not recommended. For high-temperature alkali liquid, titanium and titanium alloys or high-alloy stainless steel are often used.

Ammonia (ammonia hydroxide)

Most metals and non-metals are slightly corroded in liquid ammonia and ammonia water (ammonia hydroxide), only copper and copper alloys are not suitable for use.

Brine (seawater)

The corrosion rate of ordinary steel in sodium chloride solution, seawater and salt water is not very high, and coating protection is generally required; various types of stainless steel also have a very low uniform corrosion rate, but may cause local corrosion due to chloride ions, and 316 stainless steel is usually used.