The corrosiveness of acids

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This post was last edited by ztj1118 on 2012-12-17 at 14:39. For a long time, corrosion has been one of the most troublesome hazards for chemical processing equipment; it can cause minor damage to the equipment, or in severe cases, lead to accidents and even disasters. According to statistics, about 60% of damage to chemical processing equipment is caused by corrosion; therefore, when selecting control valves for such applications, it is essential to pay attention to the scientific choice of materials. Below are the key points for material selection when dealing with some common chemical media: 1. Sulfuric acid – As one of the highly corrosive media, sulfuric acid is an important industrial raw material with a wide range of applications. Sulfuric acid at different concentrations and temperatures causes varying degrees of corrosion to materials. For concentrated sulfuric acid with a concentration of over 80% and a temperature below 80°C, carbon steel and cast iron exhibit good corrosion resistance; however, it is not suitable for sulfuric acid that flows at high speeds, and it cannot be used as material for pumps and valves ； Common stainless steels such as 304 (0Cr18Ni9) and 316 (0Cr18Ni12Mo2Ti) also have limited applicability in sulfuric acid media. Therefore, ball valves for transporting sulfuric acid are usually made of high-silicon cast iron (which is difficult to cast and machine) or highly alloyed stainless steel (grade 20 alloy). Fluoroplastics exhibit good resistance to sulfuric acid, making the use of fluorinated-lined valves a more economical choice. 2. Acetic acid: It is one of the most corrosive organic acids; ordinary steel is severely corroded by acetic acid at all concentrations and temperatures. Stainless steel is an excellent material resistant to acetic acid, and 316 stainless steel containing molybdenum can also be used in high-temperature environments as well as with dilute acetic acid vapor. For demanding applications such as high-temperature, high-concentration acetic acid or environments containing other corrosive agents, high-alloy stainless steel butterfly valves or fluoroplastic valves can be used. 3. Hydrochloric acid: The vast majority of metal materials are not resistant to corrosion by hydrochloric acid (including various stainless steel materials); high-molybdenum high-silicon iron can only be used in hydrochloric acid at temperatures below 50°C and concentrations of less than 30%. Unlike metal materials, the vast majority of non-metallic materials exhibit good corrosion resistance to hydrochloric acid; therefore, rubber-lined valves and plastic gate valves (such as those made of polypropylene or fluoroplastics) are the best choices for transporting hydrochloric acid. 4. Nitric acid: Most metals are rapidly corroded and damaged by nitric acid. Stainless steel is the most widely used material resistant to nitric acid; it exhibits good corrosion resistance against nitric acid at all concentrations at room temperature. It is worth noting that stainless steels containing molybdenum (such as 316 and 316L) do not have better corrosion resistance to nitric acid than ordinary stainless steels (such as 304 and 321), and in some cases their resistance is even lower. For high-temperature nitric acid, titanium and titanium alloy materials are commonly used.