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topic 35405

Copper/Aluminum interaction


Q. Hi, we are currently using a thermally clad printed circuit board with aluminum base that has many high current LEDS. The aluminum underside of assembly is coated in Dow Corning 340 non curing paste and bolted to an anodized Aluminum extrusion through which water is circulated @20 degrees C. We have the option of replacing the aluminum base of thermally clad board with a copper base with improved thermal conductivity. However will I have aluminum-copper interaction problems when bolting the new copper base to aluminum extrusion.
Will the aluminum suffer corrosion?

Pauric Hennessy
lighting - Ireland


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

Guillermo Marrufo
Monterrey, NL, Mexico


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.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como
and co-author of The Sulfamate Nickel How-To Guide


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?

Bill Reynolds
Bill Reynolds
   consultant metallurgist
Ballarat, Victoria, Australia
We sadly relate the news that Bill passed away on Jan. 29, 2010.


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.

robert probert
Robert H Probert
Robert H Probert Technical Services
supporting advertiser
Garner, North Carolina
Editor's note: Mr. Probert is the author of Aluminum How-To / Aluminio El Como
and co-author of The Sulfamate Nickel How-To Guide


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?

Bill Reynolds
Bill Reynolds
   consultant metallurgist
Ballarat, Victoria, Australia


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.

trevor crichton
Trevor Crichton
R&D practical scientist
Chesham, Bucks, UK


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. Guillermo Marrufo.

Guillermo Marrufo
Monterrey, NL, Mexico


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 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.

Ted Mooney, Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
Striving to live "Aloha"

November 25, 2008

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.

Luis G. Botero
- Riverside, California, USA

July 28, 2010

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.

Jack Hunt
HVAC Consulting - Los Gatos, California, USA

June 12, 2012

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.

Thunder Lightning
- UK

July 3, 2012

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.

Terry Ethridge
Home Inspector - Kingston, Ontario, Canada

July 5, 2012

A. Very interesting, Jack. Apparently rainwater, which is of course distilled, is sufficiently non-conductive that there is no 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 wife's name is Heat :-)

Hi Terry. Thanks. Speaking as a homeowner, not as a professional, ACQ is murder. I used electrogalvanized (zinc plated) deck screws on some mixed CCA and ACQ for a deck repair and every one in the ACQ was gone (rusted completely through) in one year. The earlier types of pressure treated wood like CCA may have been more dangerous in some ways, but I'd bet that ACQ will eventually be banned as well -- and it is way way more corrosive to anything it touches than CCA was.


Ted Mooney,
Teds signature
Ted Mooney, P.E.
Pine Beach, New Jersey
Striving to live "Aloha"

February 26, 2013

Q. we have a big problem with hydrogen gas released in the radiators made of aluminium.
(some parts of wall hung boilers are made of copper.)
Some hydrogen gas is released inside the radiators and they can't get warm completely.
Can anybody say is there a way to stop this reaction, at least not to release the hydrogen gas?

Is there something to add to the water to stop this?

sarbast parvizy
hydronic heating technician - sanandaj.kurdistan.iran

March 21, 2013

Q. hi all! great forum !!! congrats!!

I would like to discuss about a possible project I am trying to design.

I would like to use aluminum tubing ( probably annealed 3003 series )
to circulate liquid ( water based with some anti-freeze, similar to car engines coolant )
to gather energy ( heat ) toward an open loop liquid reserve

I am afraid of the longevity of the system because of possible corrosion inside the aluminum tubings ( probably ~0.5" ID )

Is it very dangerous if some copper components are installed somewhere in the loop ?
( without direct contact to aluminum piping of course )
Could be in the form of a valve or a few connections.
Would represent a very small % of the total area

Also, if some copper pipes would be used as exchanger coil within the liquid reserve ?
same problems ?

Then lastly, is there any way to setup sacrificial metal in the reserve
and some environmentally friendly additives/inhibitors that you would recommend ?

Most car engines feature parts in aluminum and steel,
and some older cars even had radiators and heater cores done in copper and I don't
recall to notice a problem with galv corrosion on the alum parts or it was very small
even after 10-20 years usage with coolant.

thanks all for your time!

John Poirier
designer, interested - Montreal, Quebec, Canada

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