What are the applicable conditions for 304/304L/316/316L stainless steels in nitric acid media?
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Concentration range and temperature conditions for 304/304L resistance to nitric acid corrosion: concentration ≤ 65% (optimal 20%~65%), temperature ≤ 80°C, free of chloride ions and other halide ions. Concentration range and temperature conditions for 316/316L resistance to nitric acid corrosion: concentration ≤ 65% (optimal 20%~65%), temperature ≤ 80°C, with trace amounts of chloride ions. As a strong oxidizing acid, nitric acid promotes the formation of a dense passivation film (Cr₂O₃) on the surface of stainless steel, which is key to its corrosion resistance. This is because the stability of the passivation film is greatly affected by nitric acid concentration, temperature, and purity. Therefore, its applicability is not absolute, but is strictly limited to specific concentration and temperature ranges. For equipment involving welding or that may undergo heat treatment (450-850°C), 304L is recommended. Because after the sensitization temperature range, chromium carbide (Cr₂₃C₆) precipitates at the grain boundaries, creating chromium-deficient regions; in oxidizing acids, these chromium-deficient regions dissolve preferentially, leading to a loss of material strength and the detachment of grains. 304L possesses excellent intergranular corrosion resistance due to its ultra-low carbon content (C≤0.03%), which effectively prevents the precipitation of chromium carbide (Cr₂₃C₆) at the grain boundaries. In nitric acid media, the corrosion resistance of 316/316L is not superior to that of 304/304L; in some cases, it may even be slightly worse. The corrosion resistance of 316 is comparable to or slightly lower than that of 304. In high-temperature, high-concentration nitric acid, its corrosion rate may be higher than that of 304. Molybdenum (Mo) cannot remain stable in strongly oxidizing environments; it dissolves in the form of molybdates, slightly compromising the integrity of the passivation film. The intergranular corrosion resistance of 316L is better than that of 316, and it is comparable to that of 304L. However, in nitric acid, its corrosion resistance does not improve due to the presence of Mo. It combines the intergranular corrosion resistance of 304L with the chloride corrosion resistance of 316. In a strongly oxidizing medium such as nitric acid, the addition of molybdenum has the opposite effect. Therefore, 304/304L is usually the best economical choice in nitric acid environments (within its applicable range). The medium containing chloride ions and nitric acid is the true battlefield where 316/316L shows its advantages. Molybdenum can significantly enhance the ability of the passivation film to resist chloride ion penetration. It can tolerate trace amounts of chloride ions that may be introduced during the process (such as those from cooling water or impurities in the raw materials).
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