A4 (316) or A2 (304)? Choosing stainless fasteners for seawater

KH Marine Trading LLC · 12 November 2026

The short answer

For anything on deck, in splash zones or near seawater, A4 (316) stainless is the usual choice. A2 (304) is fine for dry, indoor or fresh-water use, but in salt spray it is more likely to show rust staining and pitting.

What makes A4 different

A4 stainless contains molybdenum, which improves its resistance to chlorides, the salt in seawater. That is why 316 is widely used for marine hardware, rigging and fasteners. Both grades are marked on the bolt head, for example A2-70 or A4-80, where the second number shows the strength class.

Watch for mixed metals

When two different metals touch in seawater, the less noble one can corrode faster. Stainless fasteners in aluminium are a common example. Insulating washers and sleeves, or a fastener material closer to the part, reduce the risk. Brass and silicon bronze fasteners are often used with bronze fittings for this reason.

Ordering checklist

State the fastener type and size, the thread (metric coarse or fine, UNC or UNF), the grade (A4-70, A4-80, A2-70, brass, silicon bronze), the quantity, and whether each bolt needs nuts and washers. For flange stud bolts, give the flange size, rating and standard.