Choosing Stainless Steel for Marine and Coastal Environments: 316 vs Duplex 2205

When stainless steel is installed near the sea, the question is rarely whether it will corrode. The real question is how much chloride (the salt in sea water and sea air) it will meet, and for how long. If you judge that correctly, the installation stays clean for decades. If you judge it wrongly, rust streaks can appear within months.

This guide explains how to match the grade, the finish, the fixings and the joints to the environment. We cover why chlorides drive the choice, why 304 falls short by the sea, why 316 and 316L are the sensible default, and when duplex 2205 is worth its cost. Then we look beyond the grade, because many coastal failures do not start in the steel sheet. They start at a crevice that traps salt water, a fixing made of the wrong metal, or a surface that nobody rinses.

Chlorides drive the choice

Stainless steel resists corrosion because of a thin chromium-oxide layer on its surface, known as the passive film. While the film is intact, it protects the steel beneath, and in clean conditions with plenty of oxygen it repairs itself when the surface is scratched. Chloride ions attack this film at one small spot rather than evenly. They break through at a weak point and stop the film from repairing itself there.

The corrosion then goes deep into one tiny point instead of spreading. That is why marine corrosion of stainless steel looks so different from the broad, flaking rust on carbon steel. It shows only as a pinhole or a brown streak, while the damage continues below the surface.

Chloride exposure is also a matter of degree, not a simple yes or no. The same handrail would behave very differently in each of these positions:

• Salty air some distance inland: mild, because airborne salt settles slowly and rain often rinses it away.
• Coastal air within sight of the sea: moderate, and enough on its own to mark a grade that is too low for the site.
• Splash and spray zones: harsh, because the surfaces are wetted again and again with concentrated salt water.
• Full immersion, or warm, still, high-chloride water: the harshest of all, where even good grades reach their limit.

A grade that performs well in one position can fail in the next, and two further factors make the attack stronger. The first is temperature: warmer water makes chloride attack much more aggressive, so a specification that lasts in a cool-climate marina may struggle in the tropics. The second is rinsing, which works in an unexpected way. A sheltered surface that rain never reaches often corrodes faster than an exposed one, because salt builds up on it instead of washing off.

So before you choose a grade, describe the environment: the distance from the shoreline, any spray or immersion, the temperature, and how often the surface is rinsed.

The corrosion types that matter at the coast

Corrosion at the coast is not one process. Five different mechanisms appear in marine work, each with its own cause and appearance. Once you know which one you are looking at, you know whether the problem is cosmetic or structural. You also know whether the fix is cleaning, a better grade or a change to the design.

• Pitting: chlorides break through the passive film at a single point and form a narrow hole into the metal. The surface may show only a small dark spot while the pit grows deeper underneath. This is the typical marine failure, and it is the reason that grades containing molybdenum exist.
• Crevice corrosion: the same process, concentrated in a tight gap where there is little oxygen and chloride builds up. Typical places are under a washer, inside a poorly sealed joint, or where two sheets overlap. It starts at lower chloride levels than pitting on an open surface, which is why crevices are so dangerous.
• Chloride stress-corrosion cracking (SCC): when tensile stress (a pulling load) and chlorides act together, cracks can grow through the metal with little warning until the part fails. It is worse at higher temperatures, and it is the failure engineers worry about most on loaded or immersed parts.
• Galvanic corrosion: when two different metals are electrically connected in salt water, the less noble metal (the one less able to resist corrosion) corrodes faster. At the coast this usually happens at fixings, where a lower-grade metal touches stainless steel and corrodes in its place.
• Tea staining: a light brown discolouration on the surface, similar in colour to weak tea, caused by mild chloride attack. It is cosmetic rather than structural, but on a hotel handrail or a prominent building it looks like neglect, and it spreads and is slow to remove.

Tea staining is the mild warning that you can see, while pitting, crevice corrosion and SCC are the serious failures, and they are often hidden. The right grade protects against pitting and SCC. The finish and regular cleaning deal with tea staining, and the design of the joints prevents crevice and galvanic corrosion. No single decision covers all five, so coastal stainless steel has to be specified as a complete system.

Why 304 usually isn't enough near the coast

Grade 304 is the most widely used grade in general stainless steel fabrication. It contains roughly 18% chromium and 8% nickel, and it is excellent value indoors and in clean environments. Near salt water it struggles, because its resistance to chloride pitting is limited, and coastal air alone is often enough to mark it.

The usual first sign is tea staining. It appears early and spreads, and it spoils the clean, lasting appearance that the client chose stainless steel for. On a very aggressive site it can also be the first stage of pitting. Grade 304 often disappoints in situations such as these:

• Coastal balustrades, handrails and signs within sight of the sea
• Exterior architectural trim exposed to salty wind
• Anything in a splash or spray zone
• Sheltered coastal surfaces that rain never rinses, where salt builds up

Near salt water, the money you save by dropping from 316 to 304 is usually spent again on cleaning, complaints or replacement.

Why 316/316L is the sensible default

For most coastal work, and for anything exposed to sea air, 316 or 316L is the sensible default, and the reason is molybdenum. Grade 316 has the same austenitic structure as 304, but it adds molybdenum, typically around 2%, which greatly improves its resistance to chloride pitting and crevice corrosion. That one change is why 316 is the standard choice for coastal buildings, handrails in sea air, deck fittings, washdown areas, and food and marine equipment exposed to salt.

The "L" in 316L means low carbon, and it solves a welding problem rather than a strength problem. When stainless steel is heated during welding, carbon can combine with chromium in the heat-affected zone. This is the narrow band of metal beside the weld that has been heated but not melted. The process is called sensitisation. It takes away some of the chromium that the steel relies on, so the metal near the weld corrodes more easily. The lower carbon content of 316L reduces this effect. So for anything that is welded and then exposed to chlorides, 316L is the safer specification, and for exposed marine work it should usually be your default.

316 is not immune, however. In permanent immersion, in warm seawater, or in crevices where chloride builds up and oxygen is scarce, even 316 can pit or suffer crevice corrosion. Those conditions are the point at which you should stop assuming 316 and start pricing duplex.

When duplex 2205 is worth the extra cost

When 316 reaches its limit, duplex 2205 is the usual next step. A simple test is this: if the part carries load, is immersed, or meets concentrated warm chloride, price duplex alongside 316L before you decide. The name "duplex" comes from its mixed microstructure, which is part austenitic and part ferritic, and that structure brings two advantages for demanding marine work.

First, it is roughly twice as strong as 316. You can often use thinner sections for the same load, which partly offsets the higher material cost and reduces the weight on cantilevered supports or long spans. Second, it resists chloride stress-corrosion cracking much better than 316.

Those two strengths show where duplex belongs:

• Structural members near or in seawater: where load and chloride act together and SCC is a real risk.
• Splash-zone supports and connections: wetted again and again with salt water while carrying load.
• High-chloride process equipment: warm, salty service that pushes 316 past its limit.
• Load-bearing parts that are permanently immersed: where a hidden crack would be both likely and serious.

It matters just as much to know when duplex is not needed. For a decorative coastal handrail or a lightly loaded screen in sea air, 316L usually gives better value, and duplex would only add cost and sometimes make fabrication more complex. Duplex is a material for a specific set of needs (strength, immersion and resistance to SCC), not a sign of higher quality.

Fixings and contact between different metals

A coastal installation often fails at its fixings rather than in its main body. The bolts, screws, rivets and brackets are where several risks meet, and they are often the first parts to corrode. Three reasons add to each other.

The first is galvanic pairing. If two different metals are connected in salt water, the less noble metal corrodes first. So a carbon-steel or lower-grade bolt in a 316 assembly can rust away while the stainless steel stays bright, staining the assembly and loosening the joint.

The second is crevice geometry. Threads, washer faces and the gap under a bolt head are the tight, low-oxygen spaces where crevice corrosion begins. So a fixing can be the right grade and still fail because of the joint it forms. The third is stress. Fixings are tightened on purpose, and that tensile load, combined with chloride, drives SCC. A highly stressed bolt in a warm splash zone is close to the worst case.

When you write the specification for the fixings, include:

• Fixing grade: fixings at least as corrosion-resistant as the parts they join. This commonly means 316 fixings for 316 assemblies, and a higher-performance option where 316 itself is only just adequate.
• Isolation: where different metals must touch, isolating washers, sleeves or coatings that break the electrical path between them.
• Drainage: joints that drain and dry instead of trapping salt water.
• Loaded connections: highly stressed connections kept out of the worst wetting, with duplex considered for loaded fixings in aggressive positions.

Making coastal stainless steel last: installation and maintenance

The grade and the fixings set the upper limit on how long an installation can last, and the joint design and maintenance decide how close you get to that limit. The same 316 handrail can look perfect after years on one building and streaked after months on another. The difference is usually design and cleaning, not the metal itself, and four points matter most:

• Design out crevices: avoid unsealed overlaps, back-to-back sections and closed pockets where salt water can collect. In a wet position, a continuous, well-made weld is often better than a lapped, bolted joint, because it leaves no crevice.
• Allow drainage: arrange sections so that water runs off rather than collecting. At the coast, standing water is salt water, and when it evaporates it leaves the chloride behind.
• Keep surfaces easy to rinse: the surfaces that corrode first are usually the ones rain never reaches, such as undersides, sheltered faces and the backs of signs. If rain cannot rinse them, a person must, so they need to be within reach.
• Choose a finish that helps: a smoother surface holds less salt and is quicker to clean, so it resists tea staining better. A well-prepared No. 4 finish (a brushed finish with fine parallel lines) suits exposed coastal work, or a mirror finish where appearance and easy cleaning both matter. A 2B mill finish (the smooth, dull-grey finish the sheet has when it leaves the rolling mill) is fine for milder exposure.

Welds need particular attention, because they are the other common weak point. Welding leaves heat tint (a coloured oxide on the surface) and a zone with less chromium near the surface, and this area corrodes first. That is why stainless steel so often rusts around welds. Pickling and passivation after welding remove that damaged layer and restore the passive film.

For any welded stainless steel going into a marine environment, write pickling and passivation of the welds into the specification. Pair this with low-carbon 316L filler and parent metal, so that the joint is no weaker than the sheet.

Finally, your project may come under the rules of a class society. These are the organisations that approve ships and offshore structures, such as DNV, Lloyd's Register, the American Bureau of Shipping (ABS) or Bureau Veritas. Their approval is a project requirement, so please name the society and the class in your first enquiry. Tell us which standard applies and we will confirm what we can provide, so that certification and procedures are planned from the start.