Honestly, if I were choosing one material for urban outdoor furniture in a coastal city, I’d put marine-grade 316L stainless steel at the top of the list. The salt air is brutal on ordinary steel and even on cheaper 304 stainless; the molybdenum in 316L gives it much better resistance to chloride pitting and rust staining. That matters for benches, frames, litter bins, bike racks, and table bases that have to sit outside for years with only occasional cleaning.
That said, I wouldn’t make the whole piece out of bare metal. For seat and back slats, I’d pair the 316L frame with UV-stabilized HDPE or good teak. HDPE is salt-proof, rot-proof, and low-maintenance, and it won’t splinter or absorb water. Teak is beautiful and durable too, but it needs more care and costs more. Aluminum is lighter and corrosion-resistant, but it can still pit or look tired if the coating fails, and galvanized steel is not something I’d trust in salt spray.
So my practical answer is: use 316L stainless steel as the structural material, specify welded joints and drainage-friendly details, and avoid trapping water. If the furniture is all one material, UV-stabilized HDPE is the low-maintenance champ for slats and seats, but it needs a strong frame. For the longest service life in a coastal urban setting, the 316L frame plus HDPE combination is hard to beat.