Honestly, when you compare cast iron vs steel street bench durability, the real difference comes down to how each material fails. Cast iron is hard, heavy, and excellent under compression, so a well-maintained cast iron street bench can sit outside for 30–50 years. Its thick sections also tolerate surface rust without losing strength quickly. The problem is that cast iron is brittle. A hard impact from a vehicle, a dropped skateboard, or vandalism can crack a leg or arm instead of bending it, and cast iron repair is not easy because it requires preheating and skilled welding.
Steel street benches are usually made from mild steel tube or bar, often hot-dip galvanized and powder coated. Steel is more ductile and has better tensile strength, so it dents or bends before it breaks. In a public setting, that is a big advantage: a bent bench is usually still standing and often repairable, while a cracked cast iron bench may collapse. The main weakness of steel is corrosion. For outdoor bench durability, coating quality is usually more important than the metal itself. If the coating gets scratched and the metal is exposed to salt, rain, or coastal air, cheap steel can rust through at the joints. But a properly galvanized steel bench can last 20–40 years or more with basic touch-up.
So in a cast iron vs steel bench comparison—or any metal bench comparison for real street use—I would pick galvanized steel for impact resistance, easier maintenance, and repairability. Cast iron is still a durable option for low-vandalism, traditional settings, but it is more brittle and less forgiving when abused.