Steel and Rust
In the last decade or so, weathering steel (brand name COR-TEN) has become a popular material for architectural and landscaping features. This material is an untreated steel alloy that will weather in the elements over time, gaining a rust-like patina. This look can provide an attractive accent in a modern landscape. That said, it is a good idea to place the steel where it will not stain concrete pavers or walls.
Weathering steel can be used for siding, fencing, planters, and even retaining walls. We have installed plenty of weathering steel planters, and we like them quite a bit. We have also seen retaining walls made of steel sheets, and they look great when installed by a qualified craftsman. When first installed, the material looks like unfinished steel that is waiting for a coat of paint. After a few seasons, it takes on the desired rusty hues. The alloy is resistant to further corrosion, but the surface looks rusty with time.
When Rust Is A Problem
The seasoning rust on weathering steel can run off onto other surfaces when exposed to excessive precipitation or sprinkler water. This may be a problem if the new concrete or paved patio is a light color that would show the rust. If weathering steel is placed in a position that allows runoff to move down a vertical wall, the color it leaves can be undesirable and more noticeable. Most of these can be cleaned with a little elbow grease, but marks left where a planter rests directly on concrete or stone can be more difficult to remove.
A mild acid is usually enough to loosen up a rusty swirl on a patio to let a nylon brush scrub the marks away. Lemon juice applied generously to the mark on a sunny day and left to sit for five to ten minutes can be enough. After the juice sits, a vigorous scrub and a quick hosing off can make the stain disappear. If necessary, another round with regular white vinegar can help, too. Be careful with your hosing, though, because even diluted vinegar can kill grass and other plants.
Really stubborn stains might require something stronger like powdered oxalic acid, also called wood bleach. This material requires gloves and extra care around plants, but it can help to remove dark rust stains with same method described above.
Weathering steel planters are more common and more popular with our clients all the time, and we really like them. However, when placed on top of a patio, they unavoidably leave some rust marks. Remember this when choosing planters for your project. If even a little rust will make you enjoy your space less, consider other options like fiberglass or powder coated aluminum.