WHICH CORROSION-RESISTANT ALLOYS WORK BEST IN MARINE APPLICATIONS?
No alloy is corrosion-proof in seawater. The right choice depends on whether the part is splashed, submerged, or just breathing salt air, and on how much the owner is willing to pay to avoid maintenance. For most marine fabrication, the shortlist is 316L stainless, duplex stainless, 5xxx-series aluminum, copper-nickel, titanium, and hot-dip galvanized carbon steel.
The biggest mistake isn't usually picking the wrong alloy. It's picking one based on the brochure rating for general corrosion and ignoring pitting, crevice corrosion, and what happens when two different metals touch.
Published September 24, 2026 · JMC Fabrication
Marine exposure breaks into zones: atmospheric (salt air, above the splash), splash (wet and dry cycling), tidal, and fully submerged. The splash zone is usually the harshest because the metal keeps getting wet with oxygen-rich seawater and then drying down to concentrated salt.
An alloy that holds up fine on a deck rail can pit through in a submerged seawater line. So the first question on any marine material decision is where exactly the part lives.
- 316L stainless steel — The default stainless for marine atmospheric and splash service. The molybdenum content gives it much better chloride pitting resistance than 304, and the L (low carbon) keeps welds from losing corrosion resistance. It can still pit and crevice-corrode in stagnant or warm seawater, so it isn't a safe choice for long-term immersion without careful design.
- Duplex and super duplex stainless — Duplex 2205 roughly doubles the yield strength of 316L and resists chloride pitting and stress corrosion cracking far better. Super duplex grades like 2507 go further and are common in seawater piping and pumps. Both need controlled heat input when welding to keep the right microstructure.
- 5xxx-series aluminum (5083, 5086, 5052) — The marine aluminum alloys. 5083 and 5086 are standard for hulls and structure that see immersion, and 5052 covers general marine sheet work. They're light and hold up well in seawater, but they're very vulnerable to galvanic corrosion when coupled to stainless or copper alloys.
- Copper-nickel (90/10 and 70/30) — A long-standing choice for seawater piping, heat exchangers, and hull fittings. Copper-nickel resists biofouling on its own, which is a big deal in intake lines. It's more expensive than steel and needs velocity limits to avoid erosion.
- Titanium — Close to immune to seawater corrosion, including in crevices and at high temperature. The cost limits it to heat exchangers, critical fasteners, and places where failure or replacement is very expensive.
- Hot-dip galvanized carbon steel — For atmospheric coastal structures, galvanizing is often the most cost-effective protection there is. Walkways, supports, railings, and framing that stay out of the water get decades from a heavy zinc coating. It isn't a long-term answer for immersion, where the zinc gets used up much faster.
For stainless steels, the Pitting Resistance Equivalent Number (PREN) is a quick way to rank chloride pitting resistance. It's calculated from the chromium, molybdenum, and nitrogen content: PREN = %Cr + 3.3 × %Mo + 16 × %N.
Typical values run around 18 to 20 for 304, 24 to 26 for 316L, about 35 for duplex 2205, and above 40 for super duplex. A common rule of thumb is that stainless for continuous seawater immersion needs a PREN of about 40 or higher. That's why 316L works on deck and struggles underwater.
When two different metals touch in seawater, the less noble one corrodes faster. Aluminum bolted to stainless is the classic example: the aluminum around the fastener corrodes, sometimes quickly. Carbon steel coupled to copper-nickel does the same thing.
Mixed-metal marine assemblies need isolation, through non-conductive washers, sleeves, and gaskets, or a design that keeps the joint dry. It's much cheaper to detail that on the shop drawings than to chase it after the first season.
- Deck hardware, rails, and topside brackets: 316L stainless or 5xxx aluminum
- Hull and submerged structure: 5083 or 5086 aluminum, or coated steel
- Seawater piping and pumps: copper-nickel, super duplex, or titanium
- Coastal structural framing and walkways: hot-dip galvanized carbon steel
- Coastal HVAC duct and exhaust: 316 stainless where galvanized won't hold up
Pick by exposure zone first, then by the corrosion mechanism that actually threatens the part, then by budget. Rank stainless options by PREN and plan for galvanic isolation on every mixed-metal joint.
JMC Fabrication fabricates stainless, aluminum, carbon, and galvanized steel for marine, shipyard, and coastal industrial work from a 50,000 sq ft shop in Pascagoula, MS, a few miles from the Gulf.
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