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Green Copper Gunk

Brian G

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Ignorant Newbie Question #487

I know I've read the answer to this before, but it won't come to me. What is that green stuff sometimes found on copper pipe or tubing? Is it a sign of any kind of noteworthy condition, or basically harmless?

Brian G.

Ignorant Newbie w/Memory Lapse [:-boggled]

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Hi Brian,

That is where some flux was left on the pipe and reacted with the pipe. In most cases it won't hurt anything, but some types of flux can continue to corrode the pipe and are supposed to be wiped off. You don't see flux spatter damage on much plumbing anymore.

For an explanation of the various type of soldering flux, go the the November Issue of Reeves Journal (p48) and read that month's technical article. You'll find it at this link.



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Most, if not all fluxes are inherently corrosive. Rosin based fluxes much less than acid base fluxes. The reason you get the green stuff near the sweat joints is the flux has accelerated the copper oxidation process by keeping fresh copper exposed. After a matter of weeks, or a few months at most if a lot of flux was left, it becomes inactive...neutral. Once the green stuff is there all the damage that flux was going to do, has been done.

Acids react but the continuing reaction will, eventually, neutralize the acid.

At any rate, I'd tell the people to wipe the joints because it's damn ugly, but it's not a defect.

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