Selection of Valves for Corrosive Media

Selection of Valves for Corrosive Media

Mesajde xinhaivalves » Mar Sep 06, 2022 9:12 am

Under working conditions with corrosive media, the anti-corrosion property is the most critical aspect of the chemical equipment. If the metal materials of the chemical valves can not be selected correctly, it’s likely to damage the equipment, and even cause accidents and disasters. So for some corrosive media, how to choose the corrosion-resistant valves properly?

1. Sulfuric Acid

As one of the strong corrosive media, sulfuric acid is an important industrial material with a wide range of applications. Sulfuric acids with different concentrations and temperatures have different corrosive effects on other materials. For concentrated sulfuric acid whose concentration is more than 80% and temperature is less than 80℃, the corrosion resistant of carbon steel and cast iron is better, but they aren’t suitable for flowing sulfuric acid at a high speed. Common stainless steel, like SS 304 (0Cr18Ni9) and SS 316 (0Cr18Ni12Mo2Ti), play a limited role in the sulfuric acid media. Therefore, the valves for transporting this medium are usually made of high silicon cast iron and high alloy stainless steel.

2. Hydrochloric Acid

Most metal materials (including all kinds of stainless steel) are not resistant to the corrosion of hydrochloric acid. And the high-silicon iron containing molybdenum can only be used for hydrochloric acid with a temperature of 50℃and a concentration of 30%. Contrary to metal materials, non-metallic materials have good corrosion resistance to hydrochloric acid so that the lined rubber and plastic pumps (e.g. polypropylene and fluoroplastic) are the best choice for conveying the hydrochloric acid. However, if the temperature exceeds 150℃ or the pressure is greater than 16 kg, all sorts of plastics are not capable of withstanding the corrosion. So at present, there is no ideal valve on the market.

3. Nitric Acid

In general, metals are rapidly corroded and destroyed in nitric acid. And the most widely used anti-corrosion material is stainless steel. However, the corrosion resistance of the stainless steel containing molybdenum (like SS 316 and SS 316L) is not superior to that of common stainless steel (like SS 304 and SS 321), or sometimes even worse. As for the high-temperature nitric acid, we usually use titanium and titanium alloy materials.

4. Acetic Acid

It is one of the strongest corrosive substances in organic acids. Common steel will be seriously corroded in acetic acids with various concentrations and temperatures, while stainless steel is an excellent anti-corrosive material. The 316 stainless steel which contains molybdenum is suitable for high temperature and dilute acetic acid steam. And for acetic acid with high temperature and concentration or other corrosive media, it is better to select valves made of high alloy stainless steel or fluoroplastic.

5. Alkali (or Sodium Hydroxide)

Steel is commonly used for sodium hydroxide of no more than 80℃ and 30% concentration. And many petrochemical plants still use common steel under the 100℃ and 75% concentration. Although the corrosion increases, the economy is good. Compared with cast iron, the corrosion resistance of common steel to alkaline has no obvious advantages. It’s not recommended to use stainless steel if a small amount of iron powder is allowed to mix with the medium.

6. Chlorine ( or Liquid Chlorine)

Most metal valves have limited effects on the corrosion resistance to chlorine. In this case, PTFE valves are good choices. However, the problem of aging will be highlighted over time. If the leakage occurs, it will be fatal. So we can replace the common PTFE lining valve with the PTFE lining ceramic ball.

7. Salt Water (or Seawater)

The corrosion rate of common steel in sodium chloride solution, seawater and salt water is not high, and we usually protect it by coating. All kinds of stainless steel also have low uniform corrosion rates, but the local corrosion may be caused by chloride ions. So it is better to use 316 stainless steel.
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