The documentation on Material reads:
CorrodedMetal 1040 Applies to
BasePartandTerrain.
This is false, however.
If you try to create CorrodedMetal terrain:
workspace.Terrain:FillRegion(Region3.new(Vector3.new(-8,-8,-8), Vector3.new(8, 8, 8)), 4, Enum.Material.CorrodedMetal)
The result is Grass which is used as a placeholder for any invalid terrain material, not Corroded Metal.
Page URL: https://create.roblox.com/docs/reference/engine/enums/Material