Tungsten carbide is what you want...£80000 a ton, £80 a kilogram.
Check out any old drill bits or cutters.It's not as magnetic as steel and does not rust.
Show us a pic of the tap thread.If it's just a normal 3/4 tap thread then just buy what i showed above but both ends female and use some ptfe tape to seal it.
Copied and pasted......Where an extension attaches to a pre-existing outbuilding,
the pre-existing outbuilding becomes part of the enlarged, improved or altered
dwellinghouse; as such the conditions in A.1 (i) apply, i.e. the resulting extension
cannot project by more than 4m from the rear wall...