by "2 or 3 cable widths" I mean that (in an ordinary domestic, office, or light-industrial environment) it is sufficient to separate the cables by a mere 2 or 3 times the outer diameter of the cable, in order to avoid thermal and EM effects. Your 18 inches is generous spacing.
on the other hand, you are working in a heavy-industrial enviroment, and that welding gear is a prime candidate for EM interference, I agree.
any metal piping will do for screening; lead, copper, steel, alum; square, round; as long as there are no "holes" for EM radiation to leak into.
so not only the piping but the junction boxes, the case enclosing the repeater, etc. must all be metal, and in good electrical contact along the full length.
here in UK, electrical regulations would also dictate that the piping be permanently bonded to an electrical earth (which helps with the screening), and it is advisable to consider lightening-surge protectors at the point(s) where the conduit enters the building. USA regs may differ.
I would recommend that you also consider other possible entry routes for this interference, e.g. :
poor quality twist where cable meets connector, causing near-end-crosstalk (NEXT)
EM interference entering via the electrical supply. Does that repeater draw power from the same distribution as the welders ? an RFI filter on the mains plug, perhaps ?
A hand-held fast-ethernet tester from such as Fluke would help to detect noise, as well as test for NEXT.
