254SMO stainless round steel (ASTM A276; UNS S31254) has a super-austenitic composition of 19.5–20.5% chromium (Cr), 17.5–18.5% nickel (Ni), 6.0–6.5% molybdenum (Mo), 0.18–0.22% nitrogen (N), 0.50–1.00% copper (Cu), ≤0.02% carbon (C), ≤0.80% silicon (Si), ≤1.00% manganese (Mn), ≤0.030% P, ≤0.010% S.
Molybdenum (Mo) and nitrogen (N) collaborate to deliver exceptional chloride resistance via two complementary chemical mechanisms:
Molybdenum's pitting inhibition: Mo (6.0–6.5%) reacts with oxygen to form molybdate ions (MoO₄²⁻) that adsorb onto the steel surface. These ions create a secondary barrier beneath the Cr₂O₃ passive layer, blocking chloride ions (Cl⁻) from penetrating and causing pitting. Mo also increases the "pitting potential" (the voltage required to initiate pitting), making it harder for Cl⁻ to break down the passive layer.
Nitrogen's passive layer reinforcement: N (0.18–0.22%) dissolves in the austenitic matrix, increasing the concentration of chromium in the passive layer. This makes the Cr₂O₃ layer denser and more resistant to Cl⁻ attack. N also forms ammonium ions (NH₄⁺) in slightly acidic environments, which neutralize local acidity around incipient pit sites-acidity accelerates pitting, so neutralization slows or stops corrosion.
Copper (0.50–1.00%) complements this synergy by enhancing resistance to sulfuric acid, while low carbon (≤0.02%) eliminates IGC risk. The combined effect gives 254SMO a pitting resistance equivalent number (PREN) of ~42, far higher than 316L's PREN of ~32.