Dielectric Unions

[caryseidner]

Are Dielectric Unions needed when joining copper to steel in a hydronic system? IRC P2904.17.1 requires them when joining copper to galvanized steel, but I could not find a reference for non-galvanized steel pipe.

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[RickSab]

My high school science (what I remember) makes me think yes.

[MTL_Inspet_Man]

whether required or not any time disimillar metals are joint one needs to take the required steps to reduce and eliminate anodic corrosion.

Even in autombiles like aston martin where steel and aluminum are used the brits wrapped the aluminum tube frame with canvas tape prior to applying the steel body panels to the frame.

In the aerospace industry the use of primers and non conducting sealants is the norm.

[kurt]

If only building codes were based in good practice like those other industries.

This is a good question. I'm interested in opinions on this one.

[Bill Kibbel]

Great question. There are other factors to consider beyond stating galvanic corrosion occurs just 'cuz steel and copper are in direct contact.

• The mass of the anodic metal compared to the cathodic metal. When the mass of the copper is smaller than the mass of the steel, the corrosion potential is greatly reduced. I find this regularly when copper tubing has been added to older, much larger steel pipe. Like your second pic.
• The electrical resistance. Pipe dope on threaded joints can create enough resistance to reduce the potential for corrosion.
• Conductivity of the water. Closed hydronic heating pipe systems likely have a less aggressive electrolyte than the domestic water system. A water softener likely increases the rate of corrosion.

[hausdok]

Hi,

I used to write those up all the time until I noticed that particular threaded copper end sweated on pipe after copper pipe connected to steel without ever showing any evidence of a reaction when I've seen a reaction with other types. I'm not sure what's up with that.

ONE TEAM - ONE FIGHT!!!

Mike

[caryseidner]

Interesting Bill. So based on the conditions shown in these photos, you don't think that a galvanic reaction will occur due to the pipe dope in photo 1, and size of the copper pipe in relation to the size of the steel pipe in photo 2. Correct?

Originally posted by inspecthistoric

• The mass of the anodic metal compared to the cathodic metal. When the mass of the copper is smaller than the mass of the steel, the corrosion potential is greatly reduced. I find this regularly when copper tubing has been added to older, much larger steel pipe. Like your second pic.
• The electrical resistance. Pipe dope on threaded joints can create enough resistance to reduce the potential for corrosion.
• Conductivity of the water. Closed hydronic heating pipe systems likely have a less aggressive electrolyte than the domestic water system. A water softener likely increases the rate of corrosion.

If the steel pipe was galvanized would dielectric unions be needed, or would these variables apply as well?

[kurt]

Well, in the noble scale, zinc (galvanizing) and copper are at opposite ends of the scale, correct? That means they have the greatest reaction, yes?

Copper and iron are closer, therefore the reaction is much less, or nonexistent, correct?

I'm grasping back about 40 years to high school chemistry, so someone fill me in here.

[Chad Fabry]

* The mass of the anodic metal compared to the cathodic metal. When the mass of the copper is smaller than the mass of the steel, the corrosion potential is greatly reduced. I find this regularly when copper tubing has been added to older, much larger steel pipe. Like your second pic.

* The electrical resistance. Pipe dope on threaded joints can create enough resistance to reduce the potential for corrosion.

* Conductivity of the water. Closed hydronic heating pipe systems likely have a less aggressive electrolyte than the domestic water system. A water softener likely increases the rate of corrosion.

What Kibbel said. He' smart. It's the reason stainless fasteners work well with aluminum castings.

Sort of related: Since mass is a huge factor, always use the more noble metal for the fastener.

[kurt]

Yeah. I'm pretty sure he's a mutant, assigned Earth duty to pave the way for invasion by higher life forms.