Three zones, three different problems

Seawater is not one environment. A structure built on the coast deals with salt carried in air; a splash-zone component deals with wetting and drying that concentrates chloride; anything permanently immersed deals with steady, warm, oxygenated brine. Each zone fails metal in its own way, which is why one grade answer never fits a marine project. The useful way to specify is to place every part in its zone first, then climb to the cheapest material that survives that zone for the life the project needs. The price gradient is steep — coated carbon steel costs a fraction of duplex, and duplex a fraction of nickel alloy — so over-specifying one zone quietly budgets away the money the next zone needed. Done in the right order, the material list usually runs from coated carbon steel at one end to nickel alloys and titanium at the other, with duplex stainless in between.

Coated carbon steel carries the coastal atmosphere

For cladding, roofing and framing set back from the spray line, the zinc layer does the work. Z275 galvanized coil carries 275 g/m² of zinc across a 0.12–2.0 mm substrate in DX51D+Z through DX56D+Z and S250GD+Z through S550GD+Z grades, and the zinc sacrifices itself so the steel underneath does not corrode. Step up to DX51D+AZ150 aluzinc and the 55% aluminium coating lasts roughly three times longer than an equivalent galvanized layer, reflects heat and tolerates over 300 °C, which is why the galvalume sheet remains the default for coastal roofs. The honest limit: cut edges and scratches expose bare steel, and chlorides attack those bare spots aggressively, so coated carbon steel belongs in the atmospheric zone only.

316L is the default — know what it is the default for

Once parts get wet regularly, fabrication moves to stainless. 316 and 316L earn the position with 2–3% molybdenum and a 0.03% carbon ceiling on the L grade, which the data sheet notes is preferred in corrosive environments and weldable without sensitisation. The 316L coil page lists the realistic duty: seawater equipment, coastal facilities, ropes, bolts and nuts — atmospheric and intermittent-wet service, not permanent immersion. Watch the substitution risk: 201 replaces nickel with manganese and costs less, but it does not carry the same chloride resistance, so a 201 coil is a false economy anywhere spray reaches. Where crevices collect, the next austenitic step is 317 with 3–4% molybdenum at 75 ksi tensile, and 317LMN adds nitrogen for 515 MPa tensile and 275 MPa yield with corrosion resistance above 316L and 317L.

Above the molybdenum ceiling

Warm, stagnant seawater defeats 6% molybdenum austenitics only in the worst crevices, but it defeats 316L routinely. 254 SMO pairs 19.5–20% chromium with 6.0–6.5% molybdenum and 0.18–0.20% nitrogen, reaches 690 MPa tensile — nearly twice the 300 series — and its own application list reads like a seawater plant: seawater cooling, ballast water systems and firefighting systems. 904L takes a different route, adding 1.2–2.0% copper to 4–5% molybdenum at 0.02% carbon, and its data sheet names sea water among its applications with intergranular resistance guaranteed in the welded condition. Both weld with ordinary shop practice; 904L simply needs the lower heat input and back-step sequencing its sheet describes, because austenitics conduct only about 30% as well as carbon steel.

Duplex grades buy strength as well as pitting resistance

Drop to about half the nickel, add nitrogen, and the yield strength roughly doubles — which lets a designer buy thinner walls. 2205 plate, stocked from 4.8 to 152.4 mm, is rated for −50 °F to +600 °F and lists cargo tanks for ships among its applications, while S31803 seamless pipe runs from 6 to 1174 mm OD with TUV, SGS, BV, ABS and LR inspection accepted. 2507 raises the alloy content to 25% chromium and 3–5% molybdenum, and the hot rolled 2507 coil names propellers, shafts, lifting equipment and hydraulic piping as its marine duty; keep it below 316 °C. S32760 / F55 is specified with PREN 40 for pitting and crevice resistance in seawater at elevated temperatures, delivers 550 MPa proof and 750 MPa tensile minimum, and holds Charpy values of 80 J at ambient and 45 J average at −46 °C.

Nickel alloys and titanium: the last stops

When the water is hot, stagnant or polluted, the nickel alloys take over. Monel 400 is the traditional marine alloy — 63% nickel minimum with roughly 30% copper, 480 MPa tensile annealed, and its data sheet claims excellent resistance to rapidly flowing brackish water and seawater, which is why marine fixtures and fasteners head its application list. It is also thoroughly standardised: plate to ASTM B127, bar to B164, seamless pipe to B165, welded with ERNiCu-7 filler. Alloy 825, titanium-stabilised so it stays as-welded, lists seawater heat exchangers and offshore oil and gas production; 625 and C-276, the latter rated for seawater and wet chlorine gas, cover the remaining hot and contaminated duties. Above them sits titanium bar: TA2/Gr2 at 99.2% minimum titanium corrodes at under 0.005 mm per year in seawater — a twentieth of 316L — at 4.51 g/cm³, and it keeps its toughness to −196 °C.

Forms, quantities and paperwork

Availability differs more across this range than price does. The 316L coil ships at 0.2–60 mm and up to 2000 mm wide in 3–10 t coils with a one-tonne minimum and ±1% tolerance. 2205 pipe and the S32205 line run seamless from 1 to 80 mm wall in lengths to 12,000 mm with 2B, pickled and polished finishes — and the same line carries 904L, 254 SMO and S32750, which keeps a mixed-alloy piping system on a single certificate. Titanium arrives as 0.5–5.0 mm sheet and bar to 80 mm under ASTM B265 and AMS 4928, minimum 100 kg, in 7–15 days. Paperwork scales with alloy value: S32760 ships with EN 10204 3.1 certification and 3.2 at surcharge, and the duplex pipe pages accept TUV, SGS, BV, ABS and LR survey. Agree the certificate level when ordering, not with the container at the port.

Rope, fasteners and the details that decide it

Secondary components sink more marine projects than the primary structure. 316 stainless wire rope is the standard choice for ship and lifting service, available in 6×19 through 6×37 constructions from 0.8 to 21.4 mm with steel or fibre core; the rope net range publishes breaking forces from 2.17 to 7.38 kN so the factor of safety can be checked rather than assumed. Detail matters more than grade: a crevice under a washer turns 316L into a corrosion cell, so specify sealed joints, continuous welds and drainage before upgrading alloy. Finally, put inspection on paper — the S31803 line accepts third-party survey — and use the cutting, bending and welding services so blanks arrive in the right alloy, in the right zone-rated grade, ready to fit.