Copper/Cast Iron Union Corrosion

[Greg Alberino]

Should I worry about corrosion between Copper/Cast Iron Union? I am retrofitting copper supply and return lines in a hydronic heating (closed) system. If so what do put between them to stop the corrosion?

[CesarParticipant]

Is there the same dielectric phenomena as with steel and copper connections. My local plumbing supply guy swears that because it is a closed system there is no need for the dielectric adaptors. I want to believe him because the pipes will be sealed in a ceiling and I would worry that the gaskets in the dielectric connectors might leak down the road leaving me with a bigger problem. If you agree that I need them, are there more permanent insulators?

» This message has been edited by Cesar on 28 August 2003

[Harold KestenholzParticipant]

Steel boilers have been plumbed with copper tubing for half a century without worry about corrosion. As long as the system is sealed without leaks, so no fresh oxygen comes into the system regularly through makeup water, there will not be the problem. There is more of a problem that can occur from improper reaming and acidic water; both of which can be avoided. Teflon tape at the threads of such bi-matal connections can add extra precaution.

[CesarParticipant]

What is improper reaming?

[Harold KestenholzParticipant]

If you just cut the tubing and leave a lip projecting into the tube, instead of cutting it off inside to the tube wall thickness, the inside lip will make the water turbulent to wear the tube out at that location.

[nicktheplumberParticipant]

I have heard this claim before, that a closed (recirculating water) system is less prone to dielectric corrosion than an open (e.g. water supply) system. This may or may not be true, but I am not convinced. The cause of corrosion in copper-iron (and that includes steel) pipe connections is the reduction/oxidation potential of the dissimilar metals. Of course there is som dissolved oxygen in the water that flows through the pipes, but I don’t think this is pertinent. More important would be the presence of dissolved ionic solids (i.e. salts) and hydronium (acid) and hydroxyl (base) ions.

Since it doesn’t really cost that much more to install dielectric unions, I would recommend using them in all installations that require joinery between ferous and cupric pipe.

NtP

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