I have a confession. The plumbing pipe I use is not recycled, but it is upcycled. Good black pipe, with all the required connections, in the size and quantity needed, is impossible to source recycled. It is a lovely versatile product, but very expensive. I get mine with screw joints to allow you to plan and build products. The screws can cause issues if you are trying to interlock and complete a square for example— you screw against yourself.
I have used the pipe in the shower room for shelves, handles and towel rails, often with scaffolding planks, and for coffee table legs. It does come exceptionally greasy and I wear gloves and use white spirit before starting any project. Otherwise, you risk staining the wood, your clothes, and anything you touch, which your partner does not appreciate!