The short answer is that it depends on the metal and how well the planter is made. Rain alone does not instantly ruin a metal urban planter box, but repeated wet-dry cycles, standing water, scratches, and contact with soil can increase rust risk. Mild steel is the most vulnerable: if it is unpainted or the coating is damaged, moisture and oxygen will start corrosion. Galvanized steel has a zinc layer that sacrifices itself first and usually performs well outdoors, though it can still rust at cut edges or after years of wear. Powder-coated steel resists rain well when the coating is intact, but chips and scratches should be touched up. Aluminum does not rust in the usual iron sense; it forms a protective oxide layer, though it can corrode in salty or very wet conditions. Stainless steel offers strong rust resistance, but lower grades may still spot in coastal or chloride-rich environments. Weathering steel, such as corten, is designed to rust; it forms a stable surface layer, but runoff can stain paving.
For a rain-exposed urban planter, drainage matters as much as material. Choose a box with drainage holes and raise it slightly off the ground so water cannot pool underneath. Avoid waterlogged soil, keep mulch away from direct metal contact if possible, and use a liner with drainage if the design allows. In coastal areas or places with road salt, rinse the planter occasionally and inspect seams, welds, and scratched edges. If you choose mild steel, expect to protect it with paint, powder coating, or galvanizing and to maintain it over time.
So, will it rust in the rain? A well-made, well-drained planter in aluminum, stainless steel, galvanized steel, or intact powder-coated steel should resist rust for a long time. An unprotected mild steel box, or any metal planter with poor drainage and damaged coating, can rust. Check the material, coating, and drainage before you buy, and maintenance will decide how long it stays attractive.