Galvanic Couple: Chem-filmed Aluminum & Gold

A. 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 Handboo (Uhlig).
if you have access to a technical library.

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 ASM Metals Handbook Vol. 1 "Properties and Selection: Irons, Steels, and High-Performance Alloys ⇦[this on on Amazonaffil links] (Eighth edition 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.

Maybe text books aren't structural members but comic books clearly are. What kid of our generation was ever happier than in a cubby hole made by throwing a sheet over tall stacks of comic books? Regards, Ted Mooney, P.E. Striving to live Aloha finishing.com - Pine Beach, New Jersey

A. Chem-filming after machining should not be a problem. It would be pretty difficult to remove the gold plating even during the cleaning process prior to chem-film. But us platers have been known to do the impossible from time to time. I would suggest you do a trial run through the entire process and communicate your intentions with your vendor. The trial run will not only prove the plating will/can hold up to the chem-film process ... it will also tell you if the plating adhesion is going to be robust enough for the post machining operation.