We often get asked if the Corten Steel Planters will stain the adjacent area by producing rust runoff or by being in direct contact with the surface the planter sits on. Below are some pictures of a Corten Planter that has been weathering in the same spot on the patio for about four months. The outside of the planter is completely covered with rust and the patina will act as a protective layer and prevent any further corrosion on the outside walls of the planter. From the pictures you can see that there is very little rust run off (virtually none). At this point the planter is weathered and the weathering steel should produce very little to no rust run off. A point to consider is that Corten Steel (weathering steel) seals and fully weathers best when it is repeatedly exposed to moisture and then is allowed to dry. Thus the amount of rust run off could vary by climate. As a reference the planter in the pictures is happily weathering in Seattle.
Also, staining will occur were the planter metal comes in direct contact with the surface the planter sits on. If you are placing your planter box on grass, lawn or dirt than there is nothing to worry about. Or if you never plan to move the planter you will never see the marks it leaves underneath the floor. But if you plant to move the planter and do not want rust marks you should make sure the planter metal does not come in direct contact with the surface that can be stained. For our planters this can be achieved by placing a plastic strips on the channel feet/legs the planter sits on. Another, solution is to place the metal planter box onto casters. Placing the planter on casters will eliminate direct contact and allow you to easily move the heavy planter.
Generally, If you cannot stand the ideal of the smallest amount of rust on your deck or patio Corten planters are probably not ideal for your application so consider other metal planter options like Stainless Steel or Powder coated aluminum.