Corrosion in sulfuric acid and material selection

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Sulfuric acid is a basic chemical raw material with high production volume and wide range of applications, and it is widely used in various sectors of the national economy. Whether it is for producing sulfuric acid or using it, any contact with it brings about the problem of sulfuric acid corrosion. Although sulfuric acid is an oxyacid, dilute and moderately concentrated sulfuric acid has weak oxidizing properties, and thus belongs to reducing acids. Concentrated sulfuric acid, especially hot concentrated sulfuric acid, has a strong oxidizing property, and therefore belongs to the category of oxidizing acids. Due to this property of sulfuric acid, when selecting sulfuric acid-resistant stainless steel, the appropriate grade varies depending on the concentration and temperature of the sulfuric acid. Generally speaking, stainless steels without Mo, such as 18-8 Cr-Ni stainless steel, cannot be used to resist sulfuric acid corrosion ； Materials such as 0Cr17Ni12Mo2, 00Cr17Ni14Mo2, and 0-1Cr18Ni12Mo2Ti, which contain 2–3% Mo, are the lowest grade materials available that offer resistance to sulfuric acid corrosion. Similarly, some ferritic-austenitic duplex stainless steels containing 2–3% Mo, such as 00Cr25Ni6Mo2N, have sulfate resistance that is comparable to or better than that of Cr-Ni austenitic steels containing 2–3% Mo. The sulfuric acid resistance of stainless steels with high Mo content (Mo ≥ 4%) is further improved compared to ordinary steels containing 2–3% Mo. However, when steel contains both Mo and Cu, its resistance to sulfuric acid is superior to that of stainless steels containing only Mo; therefore, their range of applications can be broader than that of steels containing only Mo. Cr-Ni austenitic steels and ferrite-austenitic duplex stainless steels alloyed with Mo, Cu, and Si (3–4%) are the type of sulfuric acid-resistant stainless steel that offers better performance and a wider range of applications. To date, high-grade sulfuric acid-resistant stainless steels, such as 0Cr23Ni28Mo3Cu3Ti, 1Cr24Ni20Mo2Cu3 (K alloy), 0Cr20Ni29Mo3Cu4 (Carpenter-20), etc., are mainly used to resist various concentrations of sulfuric acid at temperatures below 80ºC under atmospheric pressure. However, in high-temperature concentrated sulfuric acid at 90–98%, depending on the operating temperature and concentration, only stainless steels with a high Si content (~6%) can yield satisfactory results. Examples include the foreign patent grade SHRAMET SX (00Cr18Ni20Si6MoCu) and the domestic patent grade SS920 (ultra-low carbon, ~6% Si and containing Mo and Cu). SS920 has an annual corrosion rate of less than 0.1–0.01 in high-temperature conditions (=90% concentrated sulfuric acid). However, once exposed to dilute sulfuric acid, the corrosion resistance of high-silicon stainless steel decreases significantly.