Copper/Aluminum interaction and galvanic/hydrolysis problems

A. Hi PT. The bad news is that aluminum and copper are very highly incompatible galvanically.
But the good news is that you need both a conductive liquid path and a conductive metallic path to enable galvanic corrosion and you may have not even have either. Diesel fluid is not water and is quite non-conductive, and your gasket material seems to mean no metallic connection either (assuming there are no metal bolts through the gaskets holding the parts together).

Luck & Regards,

A. I have seen deterioration in exterior locations where light gauge sheet metal such as copper and galvanized are not physically connected and separated by six inches to several feet in a wall condition where rain water runs over them vertically and substantial corrosion occurs.

A. Answer number 3 may well be temporary, but it is certainly widely practiced in everything from automobile radiators to industrial boilers and cooling towers.

A. Paul:

There might be a third alternative. Have you considered designing your system to operate with oil, is it feasible? Many oils are almost dielectric, so galvanic corrosion would not take place or would be reduced to minimum. Hasta la vista.

I did some more research, and came up with the polymer coatings Nycote [adv: Nycote on eBay] and Topcoat. These seem like longer lasting solutions, but I would like to hear of anyone else's experiences with them. I would also like some availability information if anyone has it as I have initially had a really tough time finding suppliers for Nycote in the US (much less pricing). Thank you for your response Mr. Mooney. I still have reservations about adding an inhibitor though, as the product design calls for a solution to be more or less permanent.

Thanks again,

A. I don't exactly understand the description, Mark. But it sounds like trouble. Aluminum is a very active metal and copper a quite noble one. If both materials touch the same fluid, allowing an ionic path, and there is a metallic path as well, galvanic corrosion seems pretty likely.

A. Having a small surface area of the more noble metal certainly helps, but I don't think it makes things "okay". However, if the metals are not connected except by the water, I don't believe that galvanic corrosion is possible because you have no metallic path for electron travel.

So insulating the copper is all I need to do. Great news.

Thank you for your help.

A. I would say analyze your water before proceeding. Aluminum has an excellent oxide layer. If the level of chloride & sulfur & copper is low in water, aluminum can be used. Pitting is the issue in aluminum. Copper does better than aluminum for atmospheric corrosion but it is quite susceptible at various pH levels. Overall aluminum shows lower corrosion rates. pH for aluminum use is between 3.8 to 8.

Small copper to large aluminum sounds like a good plan as far as the area for galvanic goes.

A. Galvanic corrosion is possible if there is a metallic path (through screws). If there is not a metallic path, galvanic corrosion cannot occur. Even if it did, copper is the noble metal and aluminum is the active metal, so it is the aluminum that would corrode. Patinas, however, are tricky things; so I personally would not promise no effect on the patina if there is in fact a metallic path.

A. Any path for electricity and transfer of atoms (ions) means aluminum will be sacrificed by copper. Such a case would happen with humidity.

A. Copper is PLUS 0.34 volts OVER Hydrogen. Aluminum is negative 1.7 volts UNDER Hydrogen, that is 2.04 apart with Copper the anode, therefore copper would be sacrificed to the aluminum. However, if chlorides from salt air is around, then the aluminum will also be corroded. Properly sealed, preferable dichromate sealed, aluminum is non conductive and hence zero with respect to hydrogen.

A. Robert, I believe that, while your numbers might be correct, your conclusions are the reverse of what is correct.

Is not Guillermo's conclusion correct, and the aluminium will corrode while the copper will not corrode?

The aluminum is "anodic protected". The Anode "gives", the cathode "receives".

When you anodize 2024 the copper plates out (but may not bond), onto the cathode (aluminum or lead or stainless steel).

In this case the copper dissolves away from the higher potential and tries to plate on (and may not bond) to the lower potential.

I must continue to disagree, Robert. You're trying to turn the whole electrochemical series on its head by saying that the more noble metal corrodes while the less noble metal gives protection.

The reactions that may occur in anodising, where a large voltage/current runs through an electrolyte between two dissimilar metals that are not otherwise joined, are not relevant to the case described by Pauric Hennessy. He proposes simply clamping aluminium and copper together and wonders if there will be corrosion of one or the other. As Guillermo says, in the presence of humidity, the aluminium will corrode, as per the electrochemical series.

Will somebody else please buy into this discussion? Am I missing something that Robert sees, or is Robert seeing something that isn't actually there?

Aluminium is often used to protect copper water pipes from pitting. The aluminium is referred to as a sacrificial anode, as it will dissolve under these conditions. That makes the aluminium anodic and the copper cathodic. However, aluminium is usually covered in a protective and tenacious metal oxide that significantly shifts the oxidation potential of the "aluminium"; actually the potential in this case will be that of the aluminium oxide. Exactly how the aluminium-copper system will corrode will depend on the environment it is in. If the chemical environment promotes failure of the aluminium oxide layer, then the aluminium will corrode, if it does not, the copper MAY corrode.

? Now, I'm more confused. Is it correct that aluminum oxide is dielectric or non conductive? If so, how could it have a position in the galvanic series? The only possibility I see is that there are pores in the oxide layer produced by anodizing an aluminum surface. Someone please comment in this direction.

Ever since Mr. Probert started noting here that anodized aluminum is non conductive (which goes back a couple of years now), and which was something I always knew but hadn't really thought through, I've noticed the many outdoor applications of anodized aluminum fastened with stainless steel bolts, or u-bolted to galvanized metal, etc. And my observation has been that this seems to cause little to no galvanic corrosion problem in most cases. If the anodized coating were broken where it meets with the other metal, I guess it would be a problem. But if the anodized coating is broken elsewhere than at the connection, there still would be no metallic conductive path and apparently no galvanic corrosion. So it's good theory and born out by experience.

But as for this particular question, Mssrs. Reynolds and Crichton must surely be right: aluminum is the base metal that corrodes and copper is the noble metal that is protected.

A. Gentlemen:
From an everyday application and interaction of metals standpoint, we build MRI room [Faraday Cages.]
The most common observation has been that in the absence of special finishes, in an intentionally conductive application, the copper will always corrode the aluminum. To prevent/delay the reaction, we normally sandwich stainless components depending on the application to safe guard the aluminum leaving it now solely exposed to moisture effects.
So in magnetic fields and ground isolated rooms, the metal nobility relationship seems to always rule the day.
I hope this helps a little to clarify which metal would otherwise always corrode on contact.

A. Aluminum fins are pressure bonded to copper tubes in cooling coils. A manufacturer recently disabused me of the notion that a little bit of insulating goo was attached to the aluminum fins before the tube was expanded into a tight fit with the aluminum fin.
Water will run off of the dehumidifying aluminum and copper evaporator, and, if the identically configured coil is condensing in the rain, the entire coils will get wet, yet they will last almost indefinitely.
However if one sprays either coil with tap water, as is often done to bring down the condenser temperature on a hot day, or on the evaporator for humidification, the aluminum will rot away or oxidize into a brittle non conducting useless wafer within a year.
I don't get it.

A. I have worked in electrical distribution for many years and I can definitely say that in bi-metallic joints between copper and aluminium, the aluminium corrodes away and the copper stays intact.

A. I have inspected over 70 newer wood decks attached to aluminum sided homes which used ACQ treated wood in the deck with the results being the dissolution of the aluminum siding into fine lace. In areas of high rain water splashing the aluminum siding is proportionately dissolved.

Very interesting, Jack. Apparently rainwater, which is distilled, is sufficiently non-conductive that there is no significant galvanic action. Galvanic activity requires two metals of different potential (copper and aluminum - check), a metallic connection between them (check), and a conductive fluid in contact with both of them (tap water: check; rainwater: no check?).

Thunder: Your parents were quite the jokesters. I suppose your sister's name is Heat :-)

Hi Terry. Thanks. Speaking as a homeowner, not as a professional, ACQ is murderous. I used electrogalvanized (zinc plated) deck screws on some mixed CCA and ACQ for a deck repair and every one in the ACQ was totally gone (rusted completely through) in less than one year. The earlier types of pressure treated wood like CCA may have been more dangerous in some ways, but ACQ is way, way more corrosive to anything it touches than CCA was, and I would certainly expect that this greatly increased amount of copper leaching out is quite bad for some life forms.

Regards,

A. Hi John. You cannot (as far as I know) have galvanic corrosion without a metallic connection between the two metals. Yes, the corrosion inhibitors in automobile antifreeze are quite effective :-)

It is possible to use magnesium anodes as sacrificial anodes if desired -- they are used in home water heaters and on fresh water boats.

Luck & Regards,