304 stainless bolted to 6063 aluminum - galvanic corrosion
304 stainless bolted to 6063 aluminum - galvanic corrosion
(OP)
How big a problem will this ultimately be? I know we're set up for galvanic corrosion with these 2 materials, but I'm wondering if I'm picking at nits here and am hoping some of you with more experience in this field can offer some advice or thoughts.
We've got an assembly which consists of some 6063 aluminum angle bolted to a 304 stainless bracket w/ stainless hardware. The aluminum angle is a piece about 4 feet long which is in contact with the 304 over about a 4 square inch area. This is an outdoor application, so water will be present at times, although not constantly.
Ideally we'd get rid of the dissimilar materials, but we're kind of committed at this point due to time constraints. I know a powder coat would help the situation but not fix it entirely.
Any idea as to what kind of corrosion we're in for here? Any relatively quick and easy methods we can use to alleviate the issue?
We've got an assembly which consists of some 6063 aluminum angle bolted to a 304 stainless bracket w/ stainless hardware. The aluminum angle is a piece about 4 feet long which is in contact with the 304 over about a 4 square inch area. This is an outdoor application, so water will be present at times, although not constantly.
Ideally we'd get rid of the dissimilar materials, but we're kind of committed at this point due to time constraints. I know a powder coat would help the situation but not fix it entirely.
Any idea as to what kind of corrosion we're in for here? Any relatively quick and easy methods we can use to alleviate the issue?





RE: 304 stainless bolted to 6063 aluminum - galvanic corrosion
I'm not enough of an expert to take on the face to face corrosion problem though.
Boggs
RE: 304 stainless bolted to 6063 aluminum - galvanic corrosion
RE: 304 stainless bolted to 6063 aluminum - galvanic corrosion
Just re-read your post. Not that this is the case here, but why is stainless hardware threaded into a stainless part problematic?
Obviously I'm no corrosion expert, but I'm now in a position where the products I work on are deployed outside and I'm gonna have to get up to speed sooner rather than later. This forum is, as always, going to be a valuable resource.
RE: 304 stainless bolted to 6063 aluminum - galvanic corrosion
RE: 304 stainless bolted to 6063 aluminum - galvanic corrosion
Given time constraints, I'd say paint the mating surface and holes of the aluminum with a suitable epoxy primer, such as on QPL-23377-19. See MIL-PRF-23377J-A1 PRIMER COATINGS -- EPOXY, HIGH-SOLIDS for more info. To get both specs., enter 23377 for the Document Number at http://assist.daps.dla.mil/quicksearch/
I would also get some aluminum flat washers, yellow chromated if possible, paint them with the paint, mentioned above, and use them between the aluminum surface and the stainless washers.
If everything goes right (and no water gets trapped in the holes or between surfaces), the Al washers will be the only things that corrode for a few years, and can be readily replaced.
For more info, see MIL-STD-808A FINISH, MATERIALS AND PROCESSES FOR CORROSION PREVENTION AND CONTROL IN SUPPORT EQUIPMENT (note: identical to but more legible than the superseding MIL-HDBK-808) and MIL-STD-889B DISSIMILAR METALS (plus its Notices 1 & 3).
Also available from the ASSIST site above.
RE: 304 stainless bolted to 6063 aluminum - galvanic corrosion
I think we've made the 11th hour decision to switch the material on the Al to 304, so most of my issues go away - all of our fabbed components are now 304. I say most because there's another scenario where the piece that is now 304 stainless will be bolted to galvanized A36 steel. It appears that this is a better condition, but I'd like opinions again.
I'm continuing to try to better understand this issue - this forum is just one of the tools I'm using. Didn't want any of you to think I'm just throwing this stuff out there and waiting for answers.....
RE: 304 stainless bolted to 6063 aluminum - galvanic corrosion
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RE: 304 stainless bolted to 6063 aluminum - galvanic corrosion
If you really must mix SS fasteners with aluminum parts, there's a product called Tef-Gel that does a pretty good job of preventing both galling and corrosion. It's used fairly widely in the recreational marine industry.
RE: 304 stainless bolted to 6063 aluminum - galvanic corrosion
The members are 6061-T6 and the bolts are usually 304 stainless steel. No anodizing, no nothing. The fasteners are protected from the elements but for many applications the fasteners would be subjected directly to very high humidity under the roof (think tank full of water).
Some of these structures have been in service over 25 years and I'm not aware of any corrosion problems. Then again unless the thing fell down as a result of corrosion I don't think I would be aware.
Oh and here's an amusing tidbit, most specs actually call out 304 stainless steel fasteners as acceptable with no provision for isolation.
Maybe one of the issues here is just what is an acceptable amount of corrosion. Do you not want even one atom to leave home or can you live with a couple of mils of metal loss every 20 years?
RE: 304 stainless bolted to 6063 aluminum - galvanic corrosion
In actuality, my concern was not so much the stainless fasteners next to the aluminum, it was the fact that the aluminum piece was going to be bolted to a piece of stainless.
You did touch on my initial concern:
I can definitely live with a couple mils of metal loss every 20 years, I just wanted some more experienced points of view on whether my case would lead to corrosion significantly worse than that.
RE: 304 stainless bolted to 6063 aluminum - galvanic corrosion
Mike Halloran
Pembroke Pines, FL, USA
RE: 304 stainless bolted to 6063 aluminum - galvanic corrosion
I wonder whether we even have a galvanic corrosion issue at all given how stable (and protective) Aluminium's oxide layer is.
RE: 304 stainless bolted to 6063 aluminum - galvanic corrosion
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