In article , Grant Erwin says...

OK, check this out. Take any pipe joint you want, wrap it in teflon tape,
thread the joint together as tightly as it would be in real life, then take
it apart and see ALL the teflon tape gone from the joint. Unless it's wiping
magic invisible teflon oil onto there, it won't prevent squat for corrosion.
Tapered pipe threads seal by crushing together. The teflon tape just helps
it slide in there a little. Teflon pipe dope doesn't really stay in there
either, but it is my opinion (and those of several pro plumbers and pipefitters
I know very well) that it comes much closer than teflon tape.

The sealant (tape or dope) seals the helical gap that remains
above the crest of the male threads. If one sufficiently overpressures
an NPT fitting, the helix of teflon (or other sealant) will
extrude out of the fitting. I've seen this happen.

Jim


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