904L is a super austenitic stainless steel with approximately 25% nickel, 20.5% chromium, 4.3% molybdenum, and 1.2% copper. Its resistance to sulfuric acid results from the combined action of these alloying elements.
High nickel content lowers the corrosion rate in the active state and improves passivation in reducing acids. Copper significantly enhances resistance to dilute sulfuric acid, a property that standard 304 and 316 grades lack. Molybdenum further stabilizes the passive film and resists pitting and crevice attack.
In sulfuric acid service, 904L forms a duplex passive film consisting of an outer layer rich in iron, molybdenum, and copper oxides, and an inner chromium-rich oxide layer. This film remains stable across a wide range of acid concentrations. Practical data show that 904L handles sulfuric acid at various concentrations below 70°C, and in pure sulfuric acid from 0 to 98% at temperatures up to 40°C. In hot concentrated acid containing fluoride impurities, it outperforms 304 and even 2507 duplex steel.
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