304 stainless bolted to 6063 aluminum - galvanic corrosion

[dgowans]

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?

 

[badboggs]

I'm not sure if I read this right but I would avoid using Stainless hardware threaded into stainless. This is almost always a problem without proper precautions taken.

I'm not enough of an expert to take on the face to face corrosion problem though.

Boggs

 

[dgowans]

Sorry if my initial post wasn't clear on the hardware. The fasteners are stainless, but they are through-bolted, not threaded into the aluminum.

 

[dgowans]

Boggs,

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.

 

[israelkk]

If the bolts are threaded into the stainless parts then you are facing a galling (cold welding) between the bolt and the stainless parts resulting in a chance that you will not be able to finish bolting them or not possible to unscrew them.

 

[kenvlach]

Hope the Al 6063 was well anodized, although doubt that the holes were (Al angle is usually anodized by the extruder), and that the SS was passivated.

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.

 

[dgowans]

Thanks for all the info - it's much appreciated.

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.....

 

[EdStainless]

In general, the same rules apply. Use a good paint to seal the surfaces of the dissimilar metals prior to assembly. I would also add that using a sealer (silicone) after assembly to seal the top (and maybe sides, but never the bottom) of such joints will help. I even seal the top and sides of stainless-to-stainless joints just to minimize the risk of crevice corrosion from trapped water.

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[Cyclefrog]

BTW:
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.

 

[dozer]

Ok, I'm going to do to you corrosion guys what I hate people doing to me. Is it really that critical? I used to work for a manufacturer of aluminum structures and they have literally thousands of structures with millions of stainless steel nuts and bolts holding them together.

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?

 

[dgowans]

Dozer,

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:

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

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.

 

[MikeHalloran]

How much does its owner value whatever gets damaged when the stainless fails?



Mike Halloran
Pembroke Pines, FL, USA

 

[robsalv]

On my galvanic series, stainless is more noble than Aluminium, which means a small stainless bolt in a large aluminimum structure is the right proportion of metals to limit the galvanic corrosion.

Is it really that critical?

I wonder whether we even have a galvanic corrosion issue at all given how stable (and protective) Aluminium's oxide layer is.

 

[Metalguy]

Use an ohmeter to see if the alum. is really isolated from the stainless. The biggest problem I see is that although your anode(alum)/cathode ratio is good, any gal. corr. will take place in the alum. right next to the SS. *If* there's conduction and the metals are usually wet with rainwater, etc., the alum. will suffer as it "protects" the SS, which doesn't even need it.

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