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Jeff Wisnia Jeff Wisnia is offline
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Default Mixing metals in water pipes

dpb wrote:

Bob F wrote:

"dpb" wrote in message ...


...

Not for corrosion protection, no--the wire is a grounding wire
completing a ground around the dielectric path.



Right - but does it defeat the protection offered by the dielectric
union?



No, galvanic action is a direct contact. The ground wire, while there
is a potential yes, being dry is far less susceptible to the corrosion
and it's there where it can be seen, anyway. The dielectric between the
two water pipes themselves is still between the two dissimilar metals.
It would be nice if they were all the same material, but it's the lesser
of the evils.



I'm afraid you are wrong there, dpb. the electrical connection between
the two dissimilar metals does NOT have to be in the (wet) electrolyte area.

Just visualize a strip of copper and a strip of zinc joined together at
one of their ends and spread apart at the other. Immerse the spread ends
of the strips in a weakly acidic electrolyte, with the joined ends above
the liquid level.

Doing that effectively creates a battery, with a dead short across its
positive and negative terminals.

The zinc will corrode away pretty fast, even though the "touching" parts
aren't wet.

Jeff
--
Jeffry Wisnia
(W1BSV + Brass Rat '57 EE)
The speed of light is 1.8*1012 furlongs per fortnight.




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Cu/brass is ok...



Even when the other end of the valve is connected to galvanized
outside pipe?



Well, no, that's a different connection--it's a direct connection
between the two dissimilar metals so strictly there should be one at
each junction where switching. As noted, brass being a mostly copper
alloy is near enough in potential to not be a problem.

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