Corrosion of Dissimilar metals in HVAC coils. 1

[sduel]

We have a situation where a copper HVAC coil headder is connected to a copper nipple which is then connected to a "dielectric union". On the other side of the dielectric union is steel piping. The dielectric union was specified in order to avoid Galvanic corrosion. Unfortunately, both sides of the union are steel. Therefore, despite the fact that we seem to have electrically isolated the copper coil from the steel pipe in the rest of the system, we have a non-isolated connection between the copper nipple and the steel threads on the steel union. The concern is that the steel dielectric union will corrode. Is there any alternative to replacing the dielectric (steel to steel) union with a dielectric (brass to steel) union? Tap water runs through the coils.

 

[Cockroach]

You have the classical setup for galvanic corrosion. The exact potential could be worked out from a table showing the affinty of one metal to the other. This would also give you the expectation for anode/cathode.

There are a couple of things you could do. Probably the easiest is to try and put a insolating barrier between contact of dissimilar metals. This is usually achieved through a coat of some sort. One immediate drawback is how to coat sharp edges like those of threads where load are expected to be high on the contact areas of the flanks. I have found various titanium coats to be most effective, since adhesion is high and the surface becomes hard and resistance to wear. Corrosion resistance is not the intent here,but a beneficial result.

Other things you could try are sacrifical anodes. In this case you would place a metal of greater affinity in the cell so that the cathode would favor that reaction over your previous anodic material. This is a classical solution which has been well documented in most textbooks dealing with the subject. You could start here in search of reference literature to the phenonema.

Another thing which would mitigate but not solve completely your problem is to bring metal composition closer to eachother on the galvanic scale. This means that metal loss at the anode would be secular significant. You would therefore cater a maintenance program around this problem. Of course, when metals are exactly similar, galvanic corrosion is zero, but it is possible to have enough similarities between dissimilar metals that the resistance to the cell favors dysfunction.

Finally, try to treat the working fluid itself. This is equivalent to building up resistance in an electrical cell, thus stopping the flow of elections from cathode to anode. I hear that this rarely works, it would be considered a long shot out of despiration.

Food for thought. Hope this helps you out somewhat.

Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada

 

[sduel]

Thanks Cockroach. You've been most helpful.