Stainless Steel 321 (UNS S32100): Titanium-Stabilized — Elevated Temperature & Aircraft Exhaust

For service between 425-815°C (800-1500°F), specify 321 or 347 (niobium-stabilized). 304L relies on low carbon (<0.03%) to prevent sensitization — but at these temperatures, chromium carbide precipitation occurs within HOURS even with ultra-low carbon, consuming grain-boundary chromium and causing intergranular attack. 321's titanium carbides are thermodynamically stable and will not release carbon to form chromium carbides. However, 321 costs 20-35% more than 304L and is harder to polish or grind (TiC particles cause 'comet tails' on polished surfaces). For service below 425°C, 304L is the cost-effective choice and does not sensitize. For service above 815°C (1500°F), consider 310S (25Cr-20Ni) for oxidation resistance — 321 oxidizes excessively and forms sigma phase (FeCr intermetallic — embrittlement).

Yes — 321 has excellent resistance to nitric acid across a wide range of concentrations and temperatures. In fact, 321 is preferred over 304L for nitric acid service in the 40-70% concentration range at temperatures up to 80°C where 304L may experience end-grain intergranular attack at sheared or machined edges (the 'end-grain corrosion' phenomenon). For hot concentrated nitric acid (>90% at >50°C), 304L may transpassively dissolve — specify silicon-containing stainless (e.g., Uranus S1, DIN 1.4361) which forms an SiO₂-rich passive film resistant to oxidizing Cr(VI) species in hot HNO₃.