High-temperature and medium-concentration sulfuric acid is extremely corrosive. So far, there is no satisfactory deformable steel grade at home and abroad.
904L (N08904) is classic sulfuric acid steel. It is a low-carbon, high-nickel, molybdenum super austenitic stainless steel with good activation-passivation transformation ability.
904L has excellent corrosion resistance and can be used in any concentration of sulfuric acid at a temperature below 40 °C. When the concentration is less than 5% sulfuric acid, the operating temperature can reach 100 °C.
2. 304, 316
Type 304 stainless steel should not be used in reducing dilute sulfuric acid, and the ordinary austenitic stainless steel that can be used in dilute sulfuric acid should be at least type 316.
Compared with 304, 316 contains molybdenum, an alloying element with strong passivation ability, but at the same time, molybdenum is also a strong ferrite-forming element. In order to balance the structure, more austenite-forming element nickel is also added.
316 is corrosion-resistant (0.1mm/a) only below 30°C for 20% H2SO4. When sulfuric acid contains chloride ions, even if it is a very small amount (such as 200mg/kg), at the same 30°C, only when the concentration of H2SO4 is less than 2%, 316 is considered corrosion-resistant.
3. Ferritic stainless steel
For ferritic stainless steel resistant to concentrated sulfuric acid corrosion, Cr should be 25%~28%, Mo should be 1.5%~3.0%, and it should be alloyed with nickel, copper, titanium and niobium elements.
Ferritic stainless steel has excellent corrosion resistance in concentrated sulfuric acid, and is economical, and can be used to replace austenitic stainless steel used in pump valves, pipelines and other equipment in acid production systems.
4. High silicon stainless steel
Silicon is an inexpensive corrosion-resistant alloying element. The higher the silicon content, the more uniform, dense and thick the silicon dioxide passivation film formed on the surface of the material, thus improving the ability of stainless steel to resist general corrosion. However, the increase of silicon content will increase the tensile strength of the material, while the elongation and area shrinkage will decrease.
High-silicon stainless steels like Hastelloy D205, Sandvik SX, 700Si (2509Si), ZeCor have been successfully used in sulfuric acid production equipment such as dry suction towers, circulating tanks, acid coolers, pipelines and acid pumps. Excellent corrosion resistance.
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