Cor-ten, known in the industry as A606-4, A588, A847 or A242 is a naturally corroding metal.When the material is bundled and exposed to the elements or natural moisture that may occur, it may oxidize to a black rust/rust.
Bundled panels and or materials that are going to be stored for any length of time should be separated from each other to prevent black rust from forming on panels or materials.
If not separated and black rust forms to materials, you will not be able to remove it and it will not rust with the same look appeal as intended, nor blend in or match the rest of the material rust.