Stainless Steel Grades for Marine and…

Seawater Tests Grade Selection Like Nothing Else

Seawater combines chloride ions, oxygen, water and — in offshore service — elevated temperatures and mechanical stress: the combination that drives pitting, crevice corrosion and stress corrosion cracking. A grade that performs well in a warehouse can fail in seawater. For marine and offshore projects, grade selection decides whether the equipment reaches its design life, and the margin for casual specification is close to zero.

The difference from ordinary atmospheric service is not one of degree but of kind. Inland corrosion is mostly a slow, uniform thinning that inspection can track. Seawater attacks locally: a single pit can perforate a pipe wall while the rest of the surface still looks new, and crevice corrosion works under gaskets, bolt heads and weld roots where nobody can see it. By the time the damage is visible on a platform, it is usually already a repair job.

The Marine Grade Progression

Service life between grades in seawater spans orders of magnitude. 304 pits within weeks, 316 lasts years, and 2205 duplex lasts decades. The progression tracks the PREN (Pitting Resistance Equivalent Number), which rises with each alloying element added.

Grade	PREN	Seawater service life
304	18	Pits in weeks
316	24.5	Lasts years
2205 (duplex)	37.5	Lasts decades
2507 (super-duplex)	44	Lasts decades
6% Mo	45	Lasts decades

The step from 316 at PREN ~25 to 2205 at PREN ~37.5 is the largest jump in the table, and it is where most marine grades are decided. Below that step, you are buying time; above it, you are buying the design life. A 316 seawater line can be a defensible choice for temporary equipment or frequent-inspection service — but nobody should discover the difference between "years" and "decades" on a structure whose access requires a dive crew.

Notice also the ordering within the top of the table: 2507 and 6% Mo grades sit close together on PREN, so the choice between them is usually made on strength, availability and welding practice rather than on corrosion numbers alone.

Minimum Grades by Application

Shipbuilding structure: 316L for submerged and splash-zone components.
Offshore platforms: 2205 duplex for structural members, piping and pressure-containing equipment exposed to seawater.
Desalination plants: 2507 or 6% Mo grades for high-salinity brine and chlorinated service.
Harbour structures and piling: 316 minimum, or 2205 depending on exposure severity.
Submerged marine fasteners: 316L or A4 stainless to prevent galvanic and crevice failure.
Seawater and brine piping: 316L for seawater, 2205 for brine.

Two of these lines deserve emphasis because they are the most common source of early failures. Fasteners fail first: a bolted joint is a crevice by design, and an underspecified fastener corrodes from under the head where inspection cannot reach. And brine is not seawater — desalination concentrates salt well past ambient levels and adds chlorination chemistry, which is why the minimum grade jumps from 316L to 2507 or 6% Mo the moment a plant starts concentrating.

Order note: for splash-zone fasteners, specify A4-80 class explicitly and demand the mill test certificate chemistry — "A4 stainless" on a delivery note covers several chemistries, and only one of them is yours.

Why 2205 Duplex Dominates Offshore

2205 duplex has become the standard structural and piping grade on offshore platforms because it combines high strength with corrosion resistance. The higher yield strength allows thinner sections and lighter weight — an advantage when transporting and installing equipment on a rig, where every kilogram of steel is also a crane-hour and a deck-space decision — while a PREN of 37.5 provides the corrosion margin 316 cannot reach in seawater.

The economics compound. A thinner-wall 2205 pipe weighs less, ships cheaper, installs faster, and outlives a 316 equivalent by a factor that turns maintenance campaigns into inspections. Offshore operators did not adopt duplex out of enthusiasm for metallurgy; they adopted it because a single avoided retrofit at depth pays for the premium on the entire piping system several times over.

What to Check When Specifying

The grade name on the order is not enough. Require the mill test certificate to confirm the actual chemistry, and verify that the PREN calculated from that chemistry meets the minimum for the exposure zone. The PREN formula is a function of chromium, molybdenum and nitrogen content, so a heat running at the bottom of its band can fall below the required PREN while still being genuine, certified 2205. This is the kind of detail that separates a specification from a hope.

In aggressive conditions — hot brine, chlorinated circuits, stagnant crevices — specify super-duplex (2507) or 6% molybdenum austenitic grades and plan fabrication accordingly: these alloys want correct welding practice and qualified procedures, and the specification should say so. In our quoting experience, the buyers who attach the PREN calculation to the RFQ get heats that meet it; the ones who write only "2205" get whatever falls within the standard band.

Paying for the right chemistry at the mill costs less than replacing marine equipment early. That sentence is the entire business case for reading the certificate line by line.

Before You Freeze the Specification

Assign a minimum grade to every exposure zone — atmospheric, splash, submerged, brine — and put the table on the drawing set.
Require certificate chemistry and a PREN check against the zone minimum for every heat.
For fasteners and bolted joints, upgrade one level above the adjacent pipe; crevices fail before surfaces do.
Agree welding and pickling requirements for duplex grades in the RFQ, so fabrication capability is priced in from the start.