Mixing metals in water pipes

[Bob F]

I've done a bit of research on the subject. They say don't connect copper to galvanized. Some sites say to use a dielectric union between then. Some say brass can be used instead. I've seen dielectric unions with a hard copper wire connected to both pipes - seems like that would defeat the purpose of the union, but does code require it?

I currently have galvanized pipes with galvanized pipe coming into the house, and am planning to replace the inside pipes with copper. There is a brass valve at the entry. Do I need a dielectric union connected to that valve? Or can I just connect copper to it? Should I use dielectric unions at the water heater?

What really fails when galvanized and copper pipe are connected together? The copper? The galvanized? The joint itself?

Is it important to use copper hangers for copper pipe?

Seattle water, if it matters.

[ransley]

I think the copper wire allows the pipe to continue to be a ground, when someone used copper to galvanised on my pipe it deteriorated and looked bad fast and I replaced it, on a water heater a thermal break is needed to help to keep heat in the heater.

[dpb]

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

Cu/brass is ok...

The less noble metal is more attacked so steel (Fe) is preferentially the target. But, there is often less actual Cu physically so it may be the copper side that actually fails first. Upshot is, it can be either.

Yes.

A link that has good discussion of galvanic corrosion in water systems

-- it's specifically addressing fire protection systems but the principles are the same and it's as good/cogent discussion I've seen...

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[Bob F]

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

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

Thanks for the reference.

[dpb]

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.

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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[Jeff Wisnia]

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

[dpb]

Jeff Wisnia wrote: ...

I _said_ there's still a potential. But it's still visible for inspection and it's still the lesser of the evils -- the electrical ground has to be made.

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[Bob F]

Just like a battery with a short across its terminals? I don't understand why this is not a problem.

But the brass is not similar to the galvanized pipe screwed into it.

[Bob F]

They attach a zinc block to metal parts underwater on boats - sometimes even with a wire going to an above water connection point. This reduces corrosion of all the metal connected to it. The corrosion does not just occur at the connection point. Connect a metal from the other end of the chart the same way - everything would corrode faster, right?

[Bob F]

I am surprised the anode in the tank did not protect the nipples.

Do you suppose the the anode was similarly degraded? Or was it already gone?

[dpb]

The also make compatible metal grounding clamps.

However, I've ground connections of copper to galvanized and black iron that have been in place for 40 years or longer and they simply have not been a problem.

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[dpb]

I just go through saying that -- I was talking of the brass/copper junction. If you have a brass/galvanized on the other end, it should also be a dielectric connection.

There are compatible-metal grounding clamps available.

My experience has been however, that the grounding of a copper wire to either galvanized (scrape through to the underlying iron) or black pipe has not been a real problem in practice. It just doesn't seem to be an issue that I've observed at that point.

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[Jeff Wisnia]

I did say that I'd "changed out" (replaced) the heater, and the leak developed a few month's later. So the anode rod was new.

The adonde doesn't protect the nipples because of the galvanic current distribution through the electrolyte. It's effect can't "reach up" into the nipples.

Jeff