9Cr18Mo stainless round steel (ASTM A276; GB 10Cr17Mo) has a martensitic composition of 0.90–1.00% carbon (C), 16.0–18.0% chromium (Cr), 0.40–0.60% molybdenum (Mo), ≤0.80% silicon (Si), ≤0.80% manganese (Mn), ≤0.035% P, ≤0.030% S.
Molybdenum (0.40–0.60%) enhances pitting corrosion resistance via two chemical mechanisms:
Molybdate ion adsorption: In chloride-containing environments, molybdenum oxidizes to form molybdate ions (MoO₄²⁻) that adsorb onto the steel surface. These ions accumulate at potential pit sites (where the passive Cr₂O₃ layer is 薄弱) and block chloride ions (Cl⁻) from penetrating the layer. This prevents the initiation of pitting, as Cl⁻ is the primary ion that breaks down the passive layer.
Passive layer repair: If the Cr₂O₃ layer is damaged, molybdenum accelerates its self-healing. It increases the rate at which chromium diffuses to the surface, replenishing the passive layer faster than it can be eroded. This is critical for 9Cr18Mo, as its high carbon content (0.90–1.00%) would otherwise make the passive layer more prone to breakdown.
Chromium (16–18%) provides the base passive layer, while carbon forms hard chromium carbides that enhance wear resistance-molybdenum ensures these carbides do not compromise corrosion resistance.