Letter 2003

Galvanic Couple Between Chem-filmed Aluminum and Gold

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I think my question is simple but I can't find an answer in print. I need to know if there is a galvanic couple problem between chem filmed (Mil-C-5541 [link is to spec at TechStreet]) aluminum 6061 and gold plated (MIL-G-45204) stainless steel in an industrial atmosphere. There may be some condensation, but no salts. The final atmosphere will be space, but it may be 5 years or more before the part is launched. I have both MIL-STD-889 and MIL-HDBK-1250 but neither seem to give me a conclusive answer.

Paul Szydlowski
TRAK Microwave Corporation - Tampa, FL

Paul,

There very probably would be a galvanic couple. If you check out a table of the electromotive series of the elements (Handbook of Chem. & Physics) you will see that Al is is up near the top and Au near the bottom. In this case the Al would be anodic with respect to the Au and corrode preferentially. You might want to refer to The Corrosion Handbook (Uhlig) if you have access to a technical library.

Paul D. Stransky

I would think that the biggest problem is between the stainless steel and the aluminum, and the chromate film on the Aluminum may be effective in neutralizing the couple, as long as some of it remains.

The Metals Handbook, American Society for Metals, Vol. 1 eighth edition, Properties and Selection, page 1168 has a table of Weight loss of iron in couple combination with other metals. With aluminum, outdoor corrosion test locations around the USA, seven year weight loss for steel was lower at 7 locations when coupled with aluminum, and higher at two locations, than when not coupled with any other metal, indicating that aluminum did corrode. But that was iron, not stainless steel, and without chromate.

*** late breaking news!! In the 1948 Metals Handbook (it was all in one volume back then) (which I picked up this summer on Long Beach Island for 50 cents) (I will never sell another book from my collection again) on page 792, in the section Resistance of Al alloys to corrosion, not only has Table I electrode potentials of various metals and alloys, a section on contacts between dissimilar metals, and claims that stainless causes relatively slight attack on Al alloys since the flow of current is limited by polarization, by reference 5 which is R.B.Mears and L.J. Benson, Resistance of Aluminum-base alloys to marine exposures, Trans Soc Naval Architects and Marine Eng, 52, 91 (1944). Go get it! (this book was under the iMac, and I came this close to telling myself I shouldn't pull this thing out to look at it.) Books are not structural members of a room.

 
Tom Pullizzi
finishing.com Inc. - Brick, NJ

Did you ever get an answer on the galvanic corrosion? I have a similar interface, aluminum 6061 on gold/nickel over alloy 360 (copper zinc lead alloy, galvanic index ~0.35)

Joel Kindem
medical - Poway, California, USA

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