Replace a leaky steel pipe
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Dielectric unions stop corrosion between different types of metal by using a rubber or plastic washer and sleeve to keep the metals from touching.
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Joining copper and steel
Solder the brass half to the copper pipe, thread the steel half onto the steel pipe with Teflon tape, then join the two together tightly with the joining nut.
It's far easier to make a repair to steel pipe with copper than with galvanized
pipe, because the galvanized calls for cutting threads on exact lengths
of pipe. But first, call your local plumbing inspector or water utility and ask for
a pipe recommendation. The water in some regions is highly corrosive to copper,
making plastic (CPVC or PEX) a better choice.
Also be aware that directly joining two different types of metals, in this case,
steel and copper, can cause rapid corrosion at the joint (called dielectric corrosion).
To limit this problem, make the steel/copper connections with special
dielectric unions rather than with a regular coupling. This type
of union separates the two metals with a rubber washer and plastic sleeve so they
don't actually touch each other.
To remove the old leaky joint, turn off the water at the main entry, drain the
system and cut the pipe near the joint with a hacksaw. With a pipe wrench,
remove the pipe back to the nearest joints, working in both directions. Check the
joints to see if they're clogged or badly corroded as well. If so, consider continuing
back until you find a clean, solid fitting. Unfortunately, one bad joint often
means others or much of the system needs replacing too. Add a new galvanized
steel pipe length (nipple) at each end and solder in your copper. Then take the
union apart and solder the brass end to the copper and screw the steel end to the
galvanized pipe. Finally, join the two ends with the large nut.