For valves under corrosive media conditions, corrosion prevention is the most critical part of chemical equipment. If the metal material of chemical valves cannot be correctly selected, the equipment may be damaged at best, and accidents or even disasters may occur at 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:
1. Sulfuric acid medium valve
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°C, carbon steel valves and cast iron valves have better corrosion resistance, but they are not suitable for high-speed flowing sulfuric acid and are not suitable for use as pump valve materials; ordinary stainless steel valves 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 valves (No. 20 alloy). Fluoroplastics have good resistance to sulfuric acid, and the use of fluorine-lined valves (F46) is a more economical choice. If the pressure is too high and the temperature rises, the use of plastic valves will be impacted, and you can only choose ceramic ball valves that are much more expensive than them.
2. Hydrochloric acid medium valves
Most 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°C and 30%. In contrast to metal materials, most non-metallic materials have good corrosion resistance to hydrochloric acid, so lined rubber valves and plastic valves (such as polypropylene, fluoroplastics, etc.) are the best choices for conveying hydrochloric acid. However, if the temperature of such a medium exceeds 150℃, or the pressure is greater than 16 kg, any plastic (including polypropylene, fluoroplastics and even polytetrafluoroethylene) will not be competent. There is no ideal valve on the market yet, but you can try the emerging ceramic ball valve. The advantages of this kind of Woto valve are self-lubrication, small torque, no aging, and much longer life than ordinary valves. Its disadvantage is that the price is much higher than that of plastic valves.
3. Nitric acid medium valve
Generally, most metals are quickly corroded and destroyed in nitric acid. Stainless steel valves are the most widely used nitric acid-resistant material, and have good corrosion resistance to all concentrations of nitric acid at room temperature. It is worth mentioning that the corrosion resistance of molybdenum-containing stainless steel valves (such as 316, 316L) to nitric acid is not only not better than that of ordinary stainless steel valves (such as 304, 321), but sometimes even worse. For high-temperature nitric acid, titanium and titanium alloy materials are usually used.
4. Acetic acid medium valve
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 valves can also be used for high temperature and dilute acetic acid steam. For high temperature and high concentration acetic acid or other corrosive media, high alloy stainless steel valves or fluoroplastic valves can be used.
5. Alkali (sodium hydroxide) medium valve
Steel is widely used in sodium hydroxide solutions below 80℃ and within 30% concentration. There are also many petrochemical plants that still use ordinary steel at 100℃ and below 75%. Although corrosion increases, it is economical. Ordinary stainless steel valves have no obvious advantages 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 valves are not recommended. For high temperature alkali solution, titanium and titanium alloy valves or high alloy stainless steel valves are mostly used.
6. Ammonia (ammonia hydroxide) medium valve
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.
7. Chlorine (liquid chlorine) medium valve
Most metal valves have limited resistance to chlorine corrosion, especially when chlorine is mixed with water, including various alloy valves. In this case, PTFE valves are a very good choice, but chemical plants producing chlorine and alkali will find that: after the PTFE valve is used for a little longer, the torque will increase, and the aging problem of PTFE will become prominent. In this case, the leakage is fatal. You can consider replacing the original ordinary PTFE lined valve with a PTFE lined ceramic ball core, and the self-lubricity of ceramics and the corrosion resistance of PTFE will have a perfect effect.
8. Salt water (seawater) medium valve
The corrosion rate of ordinary steel in sodium chloride solution, seawater and salt water is not very high, and generally requires coating protection; various stainless steel valves also have a very low uniform corrosion rate, but may cause local corrosion due to chloride ions, and 316 stainless steel valves are usually better.
9. Valves for alcohol, ketone, ester and ether media
Common alcohol media include methanol, ethanol, ethylene glycol, propanol, etc., ketone media include acetone, butanone, etc., ester media include various methyl esters, ethyl esters, etc., ether media include methyl ether, ethyl ether, butyl ether, etc. They are basically non-corrosive and can be used with commonly used materials. When selecting, reasonable choices should be made based on the properties of the media and related requirements. It is also worth noting that ketones, esters and ethers are soluble in many types of rubber, so avoid mistakes when selecting sealing materials.