Concise answer
Look for the material and corrosion-resistance treatments (not just the marketing term “marine”), clear alloy or grade designations, compatible coatings or finishes, and mechanical details (threads, shank size, working/load information). Those items let you tell a genuine marine‑grade item from a general‑purpose fitting.
Useful checklist (read product descriptions and specs for these)
- Material and grade — explicit alloy or grade such as 316 stainless steel (also shown as A4), bronze or marine-grade aluminium. If a product only says “stainless” or “steel”, that’s not enough.
- Finish and treatment — passivated, electropolished, or otherwise described as treated for seawater. Look for notes on salt-spray testing or “suitable for seawater” rather than indoor use only.
- Mechanical details — working load, proof load, breaking load (or at least safe working load), thread size and type (metric vs UNC), shank/plate thickness and hole diameters. Don’t rely on vague size descriptors.
- Compatibility statements — compatibility with galvanic environments (eg use with aluminium or bronze), and whether the fitting is suitable for permanent external use.
- Fastener grade — for bolts and screws look for marine grades (eg 316) rather than generic grades (eg Grade 4.8). Again, ask if the spec only lists mechanical grade without corrosion info.
- Intended use / limitations — whether it’s rated for deck hardware, standing rigging, running rigging, hull fittings, or interior use. A “marine” label without use limits means probe further.
Why this matters
Saltwater is highly corrosive and different metals react with each other (galvanic corrosion). Marine-grade materials and treatments are chosen to resist pitting and crevice corrosion and to reduce galvanic issues. Mechanical strength is important too — a cleat or shackle must have a reliable load rating, not just look robust.
Common mistakes to watch for
- Trusting the marketing term “marine” without technical details — sellers will sometimes use it loosely.
- Assuming “stainless” equals seawater suitability — some stainless alloys are fine indoors but will pit rapidly in salt.
- Overlooking compatibility — fitting a different metal next to aluminium hull fittings can accelerate corrosion.
- Ignoring working-load info — appearance doesn’t equal rated performance.
Plain-English term notes
- Corrosion resistance: ability to resist chemical attack from saltwater.
- Pitting/crevice corrosion: localised holes or craters that form in some metals in seawater.
- Passivation/electropolishing: surface treatments that help stainless resist pitting.
Practical next step
When shopping, pick one item and compare two products’ full specs side-by-side: check the exact alloy/grade, any treatment listed, working/loading info, and whether the seller states suitability for seawater and galvanic compatibility. If any key detail is missing, ask the supplier for the alloy designation and load rating before buying.
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