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Mixing metals in water pipes Bob F 04-15-2008
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Posted by Bob F on April 15, 2008, 4:14 am
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.
http://www.ci.seattle.wa.us/util/stellent/groups/public/@spu/@ssw/documents/webcontent/spu01_002826.pdf



Posted by ransley on April 15, 2008, 8:49 am
> 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 sa=
y
> 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 hou=
se,
> 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 hea=
ter?
>
> 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.http://www.ci.seattle.wa.us/util/stellent/gro=
ups/public/@spu/@ssw/doc...

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.

Posted by dpb on April 15, 2008, 8:54 am
Bob F wrote:
> 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?

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

> 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?

Cu/brass is ok...

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

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.

> Is it important to use copper hangers for copper pipe?

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...

http://www.copper.org/applications/plumbing/techcorner/corrosion_mixed_metal_fire_sprinkler_systems.html

--

Posted by Bob F on April 15, 2008, 5:28 pm

> Bob F wrote:
>> 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?
>
> 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?

>
>> 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?
>
> Cu/brass is ok...

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

>
>> What really fails when galvanized and copper pipe are connected together? The
>> copper? The galvanized? The joint itself?
>
> 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.
>
>> Is it important to use copper hangers for copper pipe?
>
> 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...
>
>
http://www.copper.org/applications/plumbing/techcorner/corrosion_mixed_metal_fire_sprinkler_systems.html

Thanks for the reference.



Posted by dpb on April 15, 2008, 5:49 pm
Bob F wrote:
...
>> 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.

>> 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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