What Is Marine Grade Stainless Steel?

Marine grade stainless steel is the trade name for stainless steel that copes with salt air, sea spray and splashing seawater. In practice it means grade 316 or its low-carbon version 316L, and for bolts and screws the matching A4 grade under ISO 3506. No standard defines the phrase itself, so a supplier can use it loosely. The grade number on the specification is what counts.

All stainless steels resist rust because they contain at least about 10.5% chromium. The chromium forms a thin, self-repairing oxide film on the surface, called the passive layer. Chloride ions in seawater attack that film at weak points and start small pits. Grade 316 adds 2 to 3% molybdenum to the chromium and nickel of ordinary 304, and the molybdenum makes the passive layer much harder for chlorides to break down.

316L has the same composition with carbon held to 0.03% or less. Low carbon stops chromium carbides forming next to welds, so welded 316L enclosures keep their corrosion resistance through the heat-affected zone. For fabricated housings, 316L is usually the grade actually supplied.

Engineers compare stainless grades for chloride service with the pitting resistance equivalent number (PREN), worked out from the alloy content as roughly %Cr + 3.3 × %Mo + 16 × %N. A higher PREN means better resistance to pitting in salty conditions. It is a ranking tool, not a guarantee, but it shows clearly why 316 is the usual step up from 304 near the sea.

For a full side-by-side of the two common grades, including cost, machinability and where 304 is still the sensible choice, see our guide to 304 vs 316 stainless steel.

It can, which surprises people who read “marine grade” as “rustproof”. 316 resists salt far better than 304, but in a hot, salty and stagnant spot it will still show corrosion. The common problems are:

Surface finish matters almost as much as grade. A smooth, polished or electropolished surface holds less salt and moisture than a rough brushed or bead-blasted one, so it stays passive for longer. Designs that drain freely, avoid crevices and let rain wash the surface also last longer than ones with ledges where spray dries.

Regular freshwater rinsing removes the salt before it can concentrate. On deck equipment that is easy to forget, so a smooth, self-draining enclosure with flush seals is worth more than a heavier gauge of steel. Where equipment will sit in warm seawater or be hit by spray constantly, specifiers often move to duplex grades instead of relying on 316. Inspecting seals, fixings and gasket edges at each service visit catches crevice corrosion early, before it reaches the inside of an enclosure.

On a vessel or coastal site, a computer enclosure meets the same salt as the deck fittings around it. Bridge and engine room equipment is usually inside, where coated steel or aluminium is fine. Equipment on open decks, at fish processing stations, on offshore platforms or at quaysides is where 316 housings and A4 fixings earn their place, together with sealing to at least IP66 so spray cannot reach the electronics.

Our marine panel PCs and marine monitors are built for shipboard use, and our stainless steel panel PCs include fully sealed 316 versions for exposed locations. Bridge equipment is also tested to marine standards such as IEC 60945, which our guide to maritime computing requirements explains.

We supply marine and stainless steel computers and displays with the mounting, sealing and connectors matched to the location, and integrate and test each system before it ships. Tell us where the equipment will sit and how often it is washed or hit by spray, and we will recommend the grade and sealing to suit.