That's an excellent question, and it's a common concern for anyone drawn to the beautiful, warm rust patina of corten steel. The short answer is: a properly installed corten steel planter should not rust onto the surface below. Here’s the detailed explanation why.
Corten steel, often called "weathering steel," is designed to rust. However, it doesn't rust in the flaky, destructive way that traditional steel does. Instead, it forms a dense, protective layer of rust called a "patina." This patina actually seals the core of the steel, dramatically slowing down the corrosion process. Once this stable layer is formed, the steel stops actively shedding rust particles.
The real risk to the surface below isn't from the patina itself, but from two other factors: the initial "rust run-off" and water drainage.
During the first few rainfalls, before the patina fully sets, there can be some run-off of rust-colored water. This can stain light-colored concrete, stone, or wood. Furthermore, the constant moisture from watering plants inside the planter needs a place to go. If drainage is poor, water can seep out and pool underneath, leading to moisture damage and potential staining that is separate from the rust.
To completely prevent any risk of staining or damage, follow these best practices:
1. Use a Protective Barrier: Always place your corten steel planter on feet or a stand. This elevates it, allowing air circulation and preventing water pooling. Alternatively, use a durable, non-porous pad (like a composite shim or a dedicated planter pad) between the planter base and your patio or deck.
2. Ensure Proper Drainage: Confirm your planter has adequate drainage holes. It is also highly recommended to create a drainage layer at the bottom of the planter using materials like gravel, hydro granules, or broken pottery shards. This prevents the soil from becoming waterlogged and reduces the amount of water seeping from the base.
3. "Pre-Rust" Your Planter: You can accelerate the patina process yourself. Before placing the planter in its final position, expose it to several cycles of wetting and drying. This encourages the protective layer to form more quickly, minimizing the period of potential run-off.
4. Seal the Interior (Optional): For an extra layer of protection, some people choose to seal the interior of the planter with a waterproof, rust-inhibiting liner or sealant. This is especially useful for direct soil contact, further protecting the steel and controlling moisture.
In conclusion, the iconic rust of corten steel is a feature, not a flaw. With simple and correct installation—primarily by elevating the planter and ensuring good drainage—you can enjoy the stunning, earthy aesthetic of your urban planter boxes without any worry of them damaging your patio, deck, or pavement.