If you're specifying a bench for a high-pollution or acid-rain area, make 316 stainless steel your default choice for the frame, legs, and fasteners. It is not the cheapest upfront, but it is the most predictable long-term performer in corrosive urban air.
Acid rain forms when sulfur dioxide and nitrogen oxides from traffic and industry mix with moisture. That creates weak sulfuric and nitric acid, which settles on street furniture. Ordinary carbon steel will rust, zinc coatings can wear away quickly, aluminum can develop white oxide pitting if its coating is scratched, and porous materials like concrete or open-grain wood will absorb acidic moisture and hold it against metal fixings.
316 stainless earns its reputation because it contains molybdenum. That addition gives it much better resistance to pitting and crevice corrosion than 304 stainless, especially in acidic, chloride-heavy, or industrial environments. Choose a brushed, bead-blasted, or electropolished finish rather than a mirror polish. It hides scratches and the dull film that pollution leaves behind.
For seating surfaces, you have two strong options. Option one is to keep everything 316 stainless with perforated or slatted metal seats. They drain water, stay cooler in direct sun, and are easy to clean. Option two is recycled plastic lumber or a UV-stabilized fiberglass-reinforced composite. These slats do not corrode, they resist acid rain, and they are less attractive to scrap thieves than solid metal. Just specify UV-stable grades so they do not chalk or fade too quickly.
If budget or design constraints rule out 316, a good runner-up is marine-grade aluminum with a high-quality powder coat. It is lightweight and corrosion-resistant, but only if the coating stays intact. In acid-rain conditions, any deep scratch should be repaired before moisture gets underneath. Hot-dip galvanized steel with a sealed polyester powder topcoat can also work for frames, but it is a second-tier choice where pollution is aggressive.
Avoid uncoated cast iron, untreated steel, low-grade 304 stainless in coastal-industrial areas, unsealed concrete, porous stone, and solid hardwood unless it is a naturally dense species and you can maintain it. These materials either corrode, stain, or trap acidic water.
Finally, design the bench for the environment. Slope horizontal surfaces or use gaps between slats. Use stainless steel fasteners and insulating washers where different metals meet. Add drainage holes in closed sections. If you do that, a well-made 316 stainless bench in a polluted city can look decent for 20-plus years with occasional washing.