Ferralium 255 (UNS S32550) is selected for seawater service because its PREN ≥ 40 chloride pitting resistance handles ambient seawater (3.5% NaCl, ~30 °C) with substantial margin, while its 2% Cu addition gives reducing-acid resistance for downstream chemical-injection service. Standard duplex (Duplex 2205, PREN ~35) is generally limited to seawater ≤ 30 °C; austenitic 316L (PREN ~25) is unsuitable for warm seawater. Super duplex grades like Ferralium 255 extend service to warmer seawater (40+ °C), high-velocity flow regimes, and crevice-prone geometries.
Seawater corrosion attack on stainless steels is dominated by chloride-driven pitting and crevice corrosion. Resistance is predicted by Critical Pitting Temperature (CPT) per ASTM G48 method A: 316L CPT ≈ 5 °C, Duplex 2205 CPT ≈ 25 °C, Ferralium 255 CPT ≥ 35 °C, Super Duplex 2507 / Zeron 100 CPT ≈ 40 °C. PREN correlates closely with CPT, both metrics confirm super duplex grades extend the service envelope ~10–15 °C above standard duplex.
Among super duplex grades, Ferralium 255 is preferred when seawater service involves additional reducing-acid exposure, e.g., chlorination injection (creating reduced HOCl species), sulphate-reducing-bacteria (SRB) microbially influenced corrosion, or chemical injection systems carrying H2SO4, HCl, or organic acids. The 2% Cu in Ferralium 255 forms a stable Cu-rich passive film that improves performance in these reducing environments compared to Cu-free grades like Super Duplex 2507. For pure seawater service without reducing-acid exposure, all three super duplex grades (Ferralium 255, 2507, Zeron 100) perform comparably.
NORSOK M-650 QTR is typically required for North Sea and Norwegian offshore seawater service per material data sheet MDS D55. NACE MR0175 / ISO 15156-3 compliance is required when seawater combines with sour-gas exposure (downhole, wellhead, X-mas tree internals). TorqBolt provides per-heat MTC (EN 10204 type 3.2) certifying both NORSOK M-630 chemistry / ferrite-content compliance and NACE MR0175 hardness limits.
