If you are speccing benches for a coastal boardwalk or seafront promenade, salt spray changes everything. The short answer is: use 316 stainless steel for the frame and fasteners, and pair it with either marine-grade aluminum, HDPE lumber, or a genuinely salt-tolerant hardwood like ipe for the seating surface. That combination will outlast almost anything else in an urban coastal setting.
Why 316 stainless steel? Standard 304 stainless still corrodes under constant chloride exposure. 316 contains molybdenum, which gives much better resistance to pitting in salt-laden air. It is not completely immune, but it is the most practical high-durability metal for public furniture in these conditions. Use brushed or electropolished finishes rather than mirror polish, and avoid crevices where salty water can sit. Also specify 316 fasteners, not standard screws or zinc-plated hardware.
If you want a lower-maintenance seating surface, HDPE lumber is hard to beat. It does not rust, does not need painting or sealing, and will not splinter like timber. It can fade or expand in strong sun, so choose UV-stabilized grades and allow for thermal movement. Marine-grade aluminum, especially 5052 or 5083 alloys with a high-quality powder coat, is another good option for slats or supports, but any scratch that exposes bare metal should be touched up to prevent pitting.
Concrete benches are durable in coastal conditions, but the internal steel reinforcement must be epoxy-coated or stainless. Otherwise the rebar rusts, expands, and cracks the concrete from within. Fiberglass and thermoset resin benches also resist salt well, though cheaper plastic can become brittle after years of UV exposure.
Avoid plain carbon steel, cheap powder-coated mild steel, and standard zinc-plated hardware. They may look fine for a year or two, then rust streaks start. If you choose tropical hardwood, select ipe or teak and accept that it will weather to gray unless you periodically clean or oil it.
For a city procurement spec, the most defensible answer is a composite design: 316 stainless steel frame and fasteners, with HDPE or marine-grade aluminum slats, all detailed to drain water and avoid trapped saltwater. It costs more upfront, but it will look better and last far longer in salt spray than a budget steel bench.