Galvanized steel urban planter boxes are popular because they are tough, modern-looking, and naturally rust-resistant. The key is the zinc coating. When steel is galvanized, it gets a protective layer of zinc. That zinc does two jobs: it seals the steel away from air and moisture, and if the coating is scratched, it corrodes first and protects the exposed steel underneath. So, in normal use, a galvanized planter will not rust right away, and many can last for years outdoors.
That said, galvanized does not mean rust-proof forever. Rust becomes more likely when the zinc layer is damaged or used up. Common culprits include cut edges, welded seams, deep scratches, poor drainage, soil that stays soggy, salty coastal air, fertilizer salts, and acidic soil or compost. In those conditions, you may first see white rust—a chalky zinc corrosion—and later red rust if the steel itself is exposed.
The good news is that most urban planters can stay in great shape with simple care. Make sure there are drainage holes and never let water sit in the bottom. Raising the box slightly off the ground helps air circulate. Avoid harsh acids or strong fertilizers, and repair scratches with a zinc-rich touch-up paint. If you are growing edibles or using very acidic soil, a food-safe liner can reduce direct contact between soil and metal while still allowing drainage. Rinse off road salt or sea spray when you can.
Bottom line: galvanized steel urban planter boxes are rust-resistant, not rust-proof. With decent drainage, a quality coating, and a little maintenance, they can handle city gardens for a long time. If the zinc coating is thin, damaged, or constantly wet, rust will eventually find a way in.