Galvanic Corrosion with Copper/Aluminum Collectors

What are the chances that you will get galvanic corrosion with your copper tube, aluminum fin collector?

The Copper/Aluminum Collector uses copper riser tubes and aluminum fins. Since copper and aluminum are fairly widely separated in the galvanic series, there is the possibility of galvanic corrosion of the aluminum.

To get galvanic corrosion, you need 1) dissimilar metals, 2) electrical contact between the metals, and 3) a fluid path for the metal ions to travel along. If any of these three factors are not present, galvanic corrosion will not occur. The steps below are intended to keep the last two items from happening.

I recommend that the following steps be taken to reduce any chance of galvanic corrosion:

Use paint to protect the aluminum fins. The aluminum may come pre-painted, but if not paint it. A chromate primer should provide more protection.
Use the thin layer of silicone between the copper and aluminum. The silicone not only improves the thermal conductivity, but it also does not allow the copper and aluminum to touch, and (perhaps most importantly) it excludes water from the copper aluminum interface.

On the other hand, if you just take bare aluminum and form it around bare copper with nothing in the interface to separate the two or to prevent water (condensation) entry, then I'd guess its pretty likely you will get corrosion.

There is a bit of "track record" data out there on similar collectors indicating that with some precautions corrosion has not been a problem:

I've not seen any indication of corrosion in the collectors I've built.
There is at least one commercially manufactured collector that uses painted aluminum fins pressed onto copper tubes. These have been made for many years, and I've not been able to find any reports of corrosion. They address the corrosion issue in their Frequently Asked Questions...
The Maine Solar Energy Association has workshops to make solar collectors that use a similar aluminum fin and copper tube design. These workshops have been going on for many years. Again, I've not been able to find any reports of corrosion.
This is Doug's experience with rebuilding a 31 year old SunCatcher collector that uses the copper/aluminum construction...
This article describes a thermosyphon system with an aluminum fin and copper tube collector that at the time of the article had been in service for 30 years -- no corrosion. This collector is in Auckland -- a fairly humid, somewhat rainy, near ocean climate -- probably a good test.

If a small amount of corrosion should occur, the aluminum is the sacrificial metal in this pair, so it would be the one that would be "eaten", which would not cause leaks in the collector.

So, I think that with reasonable precautions galvanic corrosion is not likely to be a problem -- but, no guarantees :)

Gary July 12, 2009