Several materials resistant to sulfuric acid

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The corrosivity of high-temperature sulfuric acid at moderate concentrations is extremely strong; to date, there are no satisfactory deformable steel grades available, either domestically or internationally. I. 904L: 904L (N08904) is a classic steel suitable for use in sulfuric acid environments. It is a super austenitic stainless steel with low carbon content and high levels of nickel and molybdenum, and it possesses excellent capabilities for activation–passivation transition. 904L has excellent corrosion resistance and can be used in sulfuric acid of any concentration at temperatures below 40°C; at sulfuric acid concentrations of less than 5%, it can be used at temperatures up to 100°C. II. 304, 316: Type 304 stainless steel should not be used in reducing dilute sulfuric acid; ordinary austenitic stainless steels that can be used in such acid should be at least of type 316. Compared to 304, 316 contains the alloying element molybdenum, which has a very strong passivation ability; however, molybdenum is also a strong ferrite-forming element. To achieve structural balance, more austenite-forming element nickel is added as well. For 20% H2SO4, 316 is corrosion-resistant only below 30°C (0.1 mm/a). And when sulfuric acid contains chloride ions, even in very small amounts (such as 200 mg/kg), at the same temperature of 30°C, grade 316 is corrosion-resistant only when the concentration of H2SO4 is less than 2%. III. Ferritic stainless steels: Ferritic stainless steels resistant to corrosion by concentrated sulfuric acid should have a Cr content of 25%–28% and a Mo content of 1.5%–3.0%; alloying elements such as nickel, copper, titanium, and niobium are also required. Ferritic stainless steels exhibit excellent corrosion resistance in concentrated sulfuric acid and are cost-effective; they can be used to replace austenitic stainless steels in pumps, valves, pipes, and other equipment used in acid production systems. IV. High-silicon stainless steel: Silicon is an inexpensive corrosion-resistant alloying element. The higher the silicon content, the more uniform, dense, and thick the silica passivation layer formed on the surface of the material, thereby enhancing the stainless steel’s resistance to general corrosion. However, an increase in silicon content raises the tensile strength of the material, while reducing its elongation and area reduction. High-silicon stainless steels such as Hastelloy D205, Sandvik SX, 700Si (2509Si), and ZeCor have been successfully used in sulfuric acid production equipment including dry absorbers, circulation tanks, acid coolers, pipelines, and acid pumps, owing to their excellent corrosion resistance in high-temperature concentrated sulfuric acid.