Clean it and paint it
The pipes are galvanised and so the corrosion is zinc oxide bubbling up under the plastic coating. I've never seen one rust as the galvanising completely permeates the steel on something this size. They can get a bit pitted, but I've yet to see one bad enough to need replacing. I use scotchbrite wheels in the dremel to clean the pipe up, degrease, then a couple of coats of black enamel. You'll find the corrosion probably goes a few cm past the visible bubbles. Scotchbrite (or a fine wire brush & 400-grit paper) will take the plastic coating off anywhere there is corrosion, but where the pipe is still sound the plastic will still be stuck to it.