Something metal that sat on the slab
Rust from an object is the most common kind, and it is almost always cosmetic. A patio chair leg, a tool left out overnight, a planter with a metal base, a bike kickstand leaned against the garage. Iron in the metal reacts with water and air, and the rust that forms washes onto the concrete underneath it.
Fertilizer is a less obvious source. Some lawn and garden products carry iron to green up grass, and a bag stored on a garage floor, or drift from a spreader used near a walk, leaves the same orange stain without any metal object ever touching the concrete.
- Furniture legs and plant stands
- Tools, fasteners, or a jack left sitting out
- A rusting downspout bracket or drain grate
- Iron-based fertilizer, stored or spread nearby
Rust that comes from inside the slab
The reinforcing steel in a slab, whether that is wire mesh or a tied rebar grid, sits under concrete on every side. As long as it stays covered and dry, it does not rust. Once water reaches it, through a crack, a chip, or a spot where the steel sat too close to the surface when the slab was poured, the steel starts to corrode.
Rust takes up more room than plain steel, so as it builds it pushes outward against the concrete around it. The stain on the surface is often the first sign of that pressure, showing up well before the concrete itself cracks or pops.
Telling the two apart
An object stain holds the shape of whatever caused it and stops growing once the object is gone and the surface is cleaned. A stain coming from the steel does the opposite. It tends to run in a straight or gently curved line, following wherever that bar or that strand of mesh sits, and it often shows up right where a crack already is.
- Matches the shape of an object, does not spread: surface rust
- Runs in a line or spreads from a crack, keeps coming back: steel rust
Getting a rust stain out
Surface rust usually lifts with a concrete-safe rust remover or a poultice made for the job. Plain pressure washing rarely finishes it, because rust soaks into the pores of the concrete rather than sitting on top of it the way dirt does. Whether pressure washing is the right first step for your slab, and how hard to run it, is its own question, covered on our page on pressure washing concrete.
Once a stain is out, a penetrating sealer on that area slows the next one from soaking in as deep, though a sealer will not stop rust that keeps arriving from underneath.