Galvanic Corrosion Concern 2

[LeaderP]

We have a project with York air-handling units with chilled water cooling coils that have copper tubes, copper tube headers and steel pipe nipples for connecting the coil header to the building chilled water supply and return system.

We've challenged York about the galvanic coupling in their coil design and they've responded that since they braze the steel nipples to the copper coil header with silver alloy which is more noble than either copper or steel, they say it acts as a dielectric metal between the copper and steel.

I've never heard of using a conductive metal as a dielectric material.

Any comments?

 

[CoryPad]

BS from York.

If the connection between the two metals is electrically conductive, and if the corrosive medium can simultaneously coat both metals, the less noble will corrode.

 

[PEDARRIN2]

Correct me if I am wrong, but if you put the more noble metal between the less noble metals, the more noble metal cannot act as both anode or cathode in the corrosion reaction - thus neither the copper or steel corrode.

 

[CoryPad]

The noble metal in between does not act as an electrode, rather it is just an electron conduction path. If you place two separate metal samples into a corrosive medium and connect them with a wire, the active metal will corrode galvanically.

 

[moltenmetal]

In fact you could simply connect the copper and carbon steel parts with an EXTERNAL wire and they'd behave just the same way from a corrosion perspective.

Silver braze will not work the way they're describing.

If the chilled water system is closed loop with only infrequent make-up, it might last for a very long time due to the very limited quantity of oxygen. But if the cooling water is open loop or in contact with the atmosphere anywhere, you will lose the carbon steel material over time.

 

[PEDARRIN2]

I see what you mean - the silver braze is just the connection between the copper and steel - the redox is still betweeen the other copper and steel - the silver just completes the circuit.

 

[rmw]

As would salt water if this were a salt water based galvanic couple.

rmw