Versatile, corrosion-resistant stainless steel ideal for
structural, industrial, and fabrication applications. Available
in angles, bars, channels, flats, and beams.

Seawater presents a uniquely aggressive corrosion environment. Chloride ion concentrations of approximately 19,000 ppm, dissolved oxygen, biological fouling organisms, and the mechanical effects of wave action, abrasion, and tidal cycling combine to attack metallic materials in ways that land-based applications rarely approach.

For stainless steel, the critical concern is pitting and crevice corrosion, the localized breakdown of the passive oxide film initiated by chloride ions in low-oxygen crevice zones or at surface defects. The resistance of a stainless steel grade to this mechanism is quantified by its Pitting Resistance Equivalent Number (PREN), calculated from the chromium, molybdenum, and nitrogen contents.

Understanding PREN is the first step in selecting marine-grade materials, but it is not the only factor. Temperature, flow conditions, crevice geometry, and whether the material will be continuously immersed in the splash zone or atmosphere all significantly influence which alloy performs reliably in your application.

Marine environments are not uniform. Corrosion risk varies dramatically with the exposure zone, and the appropriate alloy selection varies accordingly.

• Atmospheric zone: Above the splash zone, with salt aerosol exposure. Grade 316L is generally suitable, provided adequate design is used to avoid crevice conditions. 304 stainless is not recommended for direct saltwater atmospheres.
• Splash and tidal zone: Alternating wet and dry with high oxygen and chloride. Duplex 2205 or higher PREN grades are typically required for reliability over extended service life.
• Fully immersed seawater: In stagnant or slow-flowing seawater with potential crevice conditions, standard 316L is not reliable. Duplex 2205, super duplex 2507, or titanium is required, depending on temperature and crevice severity.

Our specialists can help map your application’s exposure conditions to the appropriate alloy family.

• Shipbuilding and Vessels: 316L and duplex structural sections, plate, and tube for commercial vessels, workboats, ferries, and luxury yachts, including frames, decks, fittings, and exhaust systems.
• Offshore Structures: Super duplex 2507 and titanium for subsea equipment, platform structural members, riser clamps, and piping exposed to seawater at depth and elevated temperatures.
• Coastal Infrastructure: Duplex 2205 and 316L for coastal bridges, jetties, seawalls, marina fittings, wave energy structures, and architectural elements in saltwater atmospheres.
• Naval and Defense: Titanium alloys and super duplex for submarine components, naval vessel hardware, deck fittings, and defense marine equipment requiring maximum corrosion performance with minimum weight.
• Desalination: Super duplex 2507 and titanium for reverse osmosis pressure vessels, pump housings, high-pressure piping, and heat exchangers handling concentrated seawater brine.
• Marine Hardware and Fittings: 316L and 316 round bar, hex bar, and flat bar for cleats, stanchions, fairleads, rigging hardware, through-hulls, and deck hardware for recreational and commercial marine.

• PREN-Based Grade Guidance: We help customers select the minimum suitable grade for their exposure conditions, avoiding overspecification costs while ensuring reliable service life.
• Duplex and Super Duplex Stock: We stock duplex 2205 and super duplex grades in structural and bar forms, alloys that many general distributors do not carry in sufficient depth.
• Titanium in Marine Forms: Grade 2 and Grade 5 titanium are available in bar, plate, and tube for applications where even super duplex stainless steel reaches its corrosion resistance limits.
• Full MTR Documentation: Material test reports for all marine grades to support your quality and classification society documentation requirements.
• Structural Sections in Marine Grades: Angles, channels, flat bars, and beams in 316L and duplex, critical for marine structural fabrication where carbon steel is not acceptable.
• Experienced Technical Team: Our specialists understand marine corrosion mechanisms and can engage substantively on grade selection, crevice corrosion risk, and specification requirements.

Why does 316L stainless fail in seawater if it is described as marine grade?

Grade 316L is appropriately called a marine-grade stainless steel in the context of atmospheric saltwater exposure, including spray, mist, and indirect contact. In direct, sustained seawater immersion, particularly in creviced areas with low oxygen, such as under gaskets, in bolted joints, or in pipe systems, 316L’s PREN of approximately 24 is insufficient. Crevice corrosion initiation temperatures in seawater for 316L can be as low as 0 degrees C. Duplex 2205 (PREN approximately 35) or super duplex 2507 (PREN 43 and above) are required for reliable performance in immersed seawater conditions.

Is duplex 2205 suitable for fully immersed seawater service?

Duplex 2205 provides reliable performance in mildly aggressive seawater conditions, including splash zones and flowing seawater where crevice conditions are minimized. In stagnant, fully immersed seawater, particularly at temperatures above ambient, crevice corrosion risk increases, and super duplex 2507 or titanium is the recommended choice. The specific conditions of your application should be assessed carefully before specifying 2205 for fully immersed service.

Do you stock super duplex 2507 in structural forms?

We stock and can source super duplex 2507 (S32750) in bar, plate, and some structural forms. Contact us with your specific form, dimensions, and quantity to confirm current availability and lead time. Super duplex in structural sections, such as angles and channels, is less commonly stocked than bar and plate. We will advise on availability and, if needed, alternatives.

When should I specify titanium rather than super duplex for a marine application?

Titanium is the preferred choice when service conditions exceed what super duplex 2507 can reliably handle, for example, hot concentrated seawater brine above 60 degrees Celsius in desalination; the absolute minimum weight is critical, as titanium’s density is approximately 60 percent that of steel; the component is small and high-value where a material cost premium is justified; or MRI compatibility is required for marine scientific equipment. For most offshore structural applications, super duplex 2507 is the cost-effective choice. Titanium is reserved for the highest-demand thermal and immersion conditions.