Letter 35405

Copper/Aluminum interaction [Ireland] 

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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

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

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 H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

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
- Melbourne, Victoria, Australia

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 H Probert Robert H Probert Technical Services Garner, NC, USA Editor's note:        Mr. Probert is the    author of Aluminum How-To

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
- Melbourne, 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 R&D practical scientist - 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, P.E.
finishing.com Inc. - Brick, NJ

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