Galvanic Compatibility of Aluminum and Copper

A. It's true that dissimilar metal corrosion can be stopped by insulating one metal from the other, but you need to be sure of the disconnection. If you were sure that an insulator between the metals is good, then this would work.
So the chain cannot be electrically connected to the rain gutter. Perhaps a plastic link between?

A. Hi Kelly. We'll see if anyone else has a second opinion but I don't foresee any galvanic corrosion at all because hard anodizing is an excellent electric insulator.

However, you might want an o-ring to make sure your thermal paste doesn't get into the beer. And you might want to tin plate the copper because lack of galvanic corrosion does not guarantee lack of corrosion.

Regards,

A. Hi RA. One necessary step is to make sure there is no metallic connection, if that's possible to arrange, from the copper-based paint to the aluminum because that would definitely cause galvanic corrosion.

Antifoul paint is often deliberately ablative; but whether that actually means in practice that the aluminum is going to find itself surrounded by copper ions that cause it to pit, I don't personally know. Can you consider painting the aluminum saildrive and prop (not with copper based paint!), or is that a problem?

BTW, I didn't think ablative paint was required in fresh water in most cases, although low copper content paint is used :-)

Regards,

A. Hi RA,
I'm sorry to hear of this problem. I agree with Ted and there are many possibilities to investigate. Priming and painting pitted parts as depicted in the photographs is dicey at best. Much prep work such as sanding is needed and the first layer of primer needs to be applied deeply into the pits if possible. Additional primer coats are likely needed. Apply primer in a dry environment. The primer must not be left on the parts for a long time without the top coat application. Top coat should be applied immediately after final primer coat is sanded and cleaned.

Thanks for the input information.
I will be cleaning and refinishing the aluminum saildrive leg and will apply less copper based anti fouling paint in the vicinity of the saildrive leg.
I also plan to add more magnesium anodes to increase the effective surface area for sacrifice.

A. Hi Ed. Most of my roofs seemed to lose granules, which collected in the gutters. If a significant number of these granules were copper rather than aggregate, you certainly have the potential for corrosion of the aluminum. However, in your favor is that rainwater is highly non-conductive, and that the surface area of copper granules should be quite small compared to the surface area of the aluminum. I think there is a possibility of pitting of the aluminum but little likelihood of it actually being a problem. But guesses aren't the equal of experience, so you should see if any evidence-based white papers on the subject are available. Good luck.

Regards,

A. Hi Mihir. Nickel plating won't help much, but Chris Snyder says tin plating will help significantly and I believe he is right on.

Please take the time to provide the full details of your situation for better, more enthusiastic answers. Thanks!

Regards,

A. Hi Matt. Aluminum is a great material in a lot of ways, but aluminum threads are a misery. I've hacksawed aluminum hose fittings off of brass bibs, struggled with snips and pliers getting old bulbs out of lamp sockets, put an aluminum key into a lock with aluminum pins, instantly rendering it non-removable.

You can try WD-40 [on eBay or Amazon] 'ing and patiently tapping for 5 or 6 days, but I think you'll end up drilling out the brass and re-chasing the threads in the housing. There are theoretically some special acids (chromic-phosphoric) that can dissolve aluminum oxide without much damaging aluminum, but I doubt that you'd find it practical, and chromic acid is a toxic carcinogen you may not even be able to buy anyway.

Regards,

Glad to hear it, Matt. I have heard of using heat, but if I had thought of it I'd probably be a bit afraid to suggest it in a unfamiliar circumstance. Dutch Treat would be fine -- I really need to get a map and some stick pins and plan a road trip to visit the people I "meet" here :-)

Regards,

A. Hi Mike. Countless thousands of miles of aluminum-finned copper tubing has been used for baseboard heating in homes, and on home air-conditioning compressors and evaporators. If your roof-top heater lasted 20 years it's understandable that you prefer a metal replacement rather than a plastic unit.

I know nothing of this subject, but googling produced Sun Ray Solar in El Cajon as a supplier of copper/aluminum rooftop pool heaters.

Regards,

A. Hi Alfred. Galvanic current cannot flow through an insulating membrane, so you will have no galvanic corrosion. But all non-precious metals, including aluminum, corrode eventually. Presumably the door is anodized, clear coated, or powder coated to forestall corrosion.

Regards,

A. Hi Sean. Right, you cannot have galvanic corrosion between the two metals unless there is a metallic path between them to carry an electrical current.

Regards,

A. Hi Mark. Aluminum can be zincated then copper plated, which should minimize or eliminate any galvanic reaction.

Regards,

A. Hi Syed. The aluminum foil was no help, but I don't think it will be a hindrance. Aluminum is so active and copper so noble that I would not expect the aluminum foil to hurt the copper at all.

Regards,

A. The only way copper can be corroded by galvanic corrosion is if it comes in contact with a metal more noble then itself, for example Siver and Gold, Aluminum is one of the more reactive metals one can find in the construction industry, even when it develops its protective aluminum oxide layer. This means that if any galvanic coupling is created (metals coming in contact with each other) Aluminum will act as a sacrificial anode and copper will be protected.

A. Hi John. I don't think it's a good idea. Untreated aluminum is not a very decorative or corrosion resistant finish. And though I'm not sure exactly how the high copper content in the treated lumber will leach onto the adjacent aluminum or interact with it, it can't be good :-(

If you search the site for "clearcoat aluminum" you'll find several very good dialogs on the subject. Good luck.

Regards,

A. Hi Ricardo. You can't have galvanic corrosion without an electrolyte (a conductive liquid).

Regards,

A. Hi Jerry. Neither aluminum nor zinc is compatible with copper, but painted aluminum ought to be inexpensive and last a reasonable time.

Regards,

A. Hi Jerry,
Have you considered a physical insulating barrier between the two metals where they touch, such as a strip of semi-rigid plastic, in case there is any abrasion and the paint scrapes off the aluminum, or the aluminum is only painted on the outside? Also, can you use nonmetallic fasteners such as Acetal bolts if you are joining two dissimilar metals? You should read up on the strategies they've used on the Statue of Liberty restoration; it's pretty neat. As I recall, PTFE-sealed bolts and washers are involved, with the intention of breaking any possible electrical conductivity that can lead to corrosion.

A. Hi Rachel. For many applications I think your solution will work, but copper roofs usually have copper actually running off of them, as we can see from the green stains. I think the copper salts/oxides will be in the runoff and in the gutters.

Regards,

A. Hi Taidgh. Please try to give us the whole story so we can help you best, rather than an extract covering a single step. Originally I simply wrote that I agreed with you that iron phosphating a copper-aluminum item sounded like a bad idea to me as well ... but then I realized the possibility that maybe you want to do this iron phosphating step because you are considering e-coating the coil assembly? In that case, yes, you must pretreat the aluminum.

I don't think putting a copper and aluminum article into a 5-stage iron phosphate line will seriously contaminate the process, but I'd need to see the tech data sheets from the process supplier to be confident about it. Sometimes there are chemicals put into a process to optimize it for a particular application which are inappropriate for a more general application. The phosphating process can and should be optimized for processing aluminum :-)

(Readers: Using the google search term "aluminum-fin copper corrosion" produces a good deal of interesting papers covering theory, practical solutions like pretreating the aluminum vs. e-coating the assembly, and the use of corrosion preventative compounds for this particular application).

Regards,

  Ted, thank you for your response. This particular product is a copper tube with an aluminum fin that are both attached to a cold rolled steel frame. Unlike a typical fin and tub heating element these are not enclosed and will be powder coated, which is why they are being pretreated. The components must be assembled prior to powder coating. Unfortunately, I cannot provide the wash chemistry as it is all listed as "trade secret" on the data sheets. I will reach out to the supplier to see if they have any reason to suspect an adverse effect of washing these materials together. Thank you for your assistance.

A. Hi Mathew.
Zinc vs. copper is about as bad as galvanic corrosion gets. The electrochemical difference is over a volt, and many school kids are familiar with making batteries by inserting a copper penny and a galvanized nail into a lemon to light up LEDs and power digital clocks.

It is is tough to offer concrete answers about galvanic corrosion because it also depends on the conductivity of the moisture (very low if only rainwater, but much higher is salt or fertilizer lands on the box). And because we can talk about relative areas, but the only area that counts is the conductively wetted area :-(
If the plate is vertical and sheds water so it's usually dry, but conductive glop accumulates under the joint, the area relationship between the same parts may be very different than if the plate is horizontal so it tends to remain wet.

So practical good practices may be better than calculation, and what might prove to be inexpensive, practical, and a big help is to use tin plated terminals and tin plated copper or brass rivets.

Regards,

A. Hi Richard. It doesn't require much to block those galvanic currents; black electrician tape would be more than enough. But I don't know what tape adhesive is going to last decades with such exposure. It may not be a big issue since rainwater is not very conductive and the covers won't be very visible. So if the tape is wedged in where it won't fall out and promote blockage, it's probably all you need. But "plater's tape", which is a bit heavier and available in colors, might be good.

Luck & Regards,

A. Hi Betty. Presumably the aluminum roofing is painted, which will at least minimize the staining from copper runoff. If it is practical for the copper to be lacquered or clearcoated, that would help too.

Luck & Regards,