Galvanic Couple: Chem-filmed Aluminum & Gold
An ongoing discussion from 1998 through 2015 . . .(1998)
Q. 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-DTL-5541 [link is to free spec at Defense Logistics Agency, dla.mil]) aluminum 6061 and gold plated ( Mil-G-45204 [linked by editor to free spec at Defense Logistics Agency, dla.mil]) 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 [link is to free spec at Defense Logistics Agency, dla.mil] and MIL-HDBK-1250 but neither seem to give me a conclusive answer.Paul Szydlowski
microwaves Tampa, Florida
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.
- Putnam, Connecticut
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" [link is to info about book at Amazon] (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?
Ted Mooney, P.E. RET
Pine Beach, New Jersey
Q. 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
Need Military Avionics Selective Gold Plating/Chem Film(2005)
I need selective gold plating on 6101-T6 with high accuracy?
Non-plated surfaces are chem filmed.
Our current plating shop has difficulty delineating and leaves chem film stains on the gold.
This is for a Military Avionics Application; could be a significant piece of business.
Quantities will start low, then build over a period of several years.
I can send a drawing.
Military Avionics Manufacturer - Palm Bay, Florida, USA
^- Sorry, this RFQ is outdated
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Will Gold Plating Withstand Chem Film Processing?April 29, 2015
Q. We are trying to eliminate masking to save cost on the finishing requirements on our products. Our typical base metal is 6061 aluminum, with a gold over nickel finish. However, we have many features that must be bare and or chem filmed. Does the chem film-process have any effect on the gold finish surface? Ideally we would like to gold plate the entire part, post drill and mill any areas that require chem film and then drill and mill again for bare aluminum. Please tell me if the gold would be affected by the process.Roman Burtyk
Product designer - Chandler, Arizona usa
May 4, 2015
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.
Foresight Finishing LLC
- Tempe, Arizona
Chem film durability in contact with goldNovember 20, 2015
Q. I have 100 micro inches of gold over nickel on a 6061 housing. It needs to be bolted down to a 6061 baseplate that has been chem filmed type 1, class 3.
The concern is the act of mechanical ass'y will abrade the film off and bring Au in contact with Al leading to galvanic corrosion. I'm not convinced this is a problem but cannot find any good info on the durability of the finish. Bottom line, if care is taken when bolting down the unit and a chem film surface comes in contact with a gold surface, is there a problem?
Design engineer - Chandler, Arizona USA
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