Yes—in most outdoor urban settings, an ungalvanized steel bench can start showing visible rust within weeks to a few months, and it can develop serious structural corrosion within a couple of years if left unprotected. Bare carbon steel has no natural corrosion barrier, so when the iron in the steel meets oxygen and water, it forms iron oxide. Rain, morning dew, high humidity, road salt, bird droppings, and air pollution all speed up that reaction.
The exact speed depends on the finish and the environment. A bare, ungalvanized steel bench in a rainy coastal city may show surface rust in days. In a dry inland climate, the process is slower but still inevitable. If the bench has a good powder coat or paint system, the coating can protect the steel for years, but any scratch, chip, or bolt hole that exposes the metal becomes a rust starting point. Once rust begins under paint, it can spread beneath the coating and cause blistering or flaking.
Galvanizing changes the picture because the zinc layer acts as a sacrificial anode. Even if the coating is scratched, the surrounding zinc corrodes first and protects the underlying steel. That is why many urban benches use hot-dip galvanized steel under a painted or powder-coated finish. It costs more upfront, but it dramatically extends service life.
If you are choosing a bench for a park, street, plaza, or schoolyard, ungalvanized steel is usually a poor choice unless you live in a very dry climate and are diligent about repainting scratches immediately. A better specification is hot-dip galvanized steel, stainless steel, or galvanized steel with a tough polyester powder coat. These options resist moisture, salt, and everyday abuse much better.
So, would an ungalvanized steel urban bench rust quickly? Yes, especially outdoors. You might get away with it indoors or in a protected alcove, but for public outdoor furniture, ungalvanized steel usually means visible rust early and more maintenance later.