COR-TEN is a trademark of the United States Steel Corporation. The idea behind this striking material is that a surface layer of rust will form, protecting the steel underneath from further deterioration and eliminating the need for paint. However, the development of this layer requires particular environmental conditions which can’t be guaranteed.
When the metal gets wet and the dries out, the rust layer becomes a little thicker, while the underlying solid steel becomes thinner. In an ideal world, the rust layer would continue to take hold of the steel for two or three years before stabilizing. The result should be a maintenance-free material with a unique aesthetic.
But in the real world, different conditions like too much humidity often mean that the rust layer never reaches a stable point. Eventually, the metal can become perforated and may need to be replaced.