Corrosive properties of Stainless Steel contacting aluminum 6061 4

[rocketscientist628]

During some recent "re-work", a fellow engineer at our company discovered corrosion on stainless steel screws that were removed from threaded holes that were tapped in aluminum 6061. MOST all of the hardware we build is machined out of 6061, and most ALL the fasteners we use are stainless steel. Upon his discovery, we have adopted the policy of coating ALL contact areas of stainless and 6061 including threads and fastener head contact areas with zinc chromate primer. However, it is not always "convenient" to use this. We are looking for other alternatives. Has anyone else discovered this corrosion between stainless and 6061? We realize that we could eliminate the problem if we had cadmium coated screws, but we also realize how difficult they are to procure. Any ideas on alternate methods of approach to solve this problem? By the way, the 6061 is typically sent thru a chromate conversion (iridited) such as Mil-C-5541.

 

[CoryPad]

Stainless steel and aluminium alloys can have galvanic corrosion between them. Their passive oxides can mitigate this problem, but it doesn't always work. How about aluminium alloy screws? Or, you can use carbon steel fasteners with appropriate coatings (Doerken, Magni, Metal Coatings). If you are going to keep stainless steel screws, then maybe you should use an organic coating on the aluminium piece.

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mcguire]

I would look elsewhere for the source of the corrosion since aluminum should be galvanically protecting the stainless. Do you possibly have iron contamination on the screws? Can you describe the nature of the corrosion that was found?

 

[EdStainless]

Are these exposed to a marine environment?
My guess is that it is crevice corrosion. The Al corrodes slightly, then the Al oxide traps moisture and chlorides, and then the stainless corrodes.
You might have better luck sealing the joints.

= = = = = = = = = = = = = = = = = = = =
Corrosion never sleeps, but it can be managed.

http://www.trenttube.com/Trent/tech_form.htm

 

[rocketscientist628]

Thanks for all your help!

 

[monkeydog]

Rocketscientist628,
So, what was the final conclusion? A marine environment? It hardly seems that given the choice for corrosion to work it would go after CRES when their is tasty Al in the same area.

 

[israelkk]

I am no corrosion expert. However, in a similar problem in marine environement we swithced to corrosion resistance A-286 alloy bolts and fasteners. The 6061 was chromic anodize coated or sometimes hard anodize coated per MIL-A-8625 which is much superior to just conversion coating per Mil-C-5541 which will degrade after long axposures to temperature higher to than 60 Celsius. You may consult MIL-STD-889 "Dissimlar metals" too which can be found at

http://assist.daps.dla.mil/eAccess/index.cfm?ident_number=36032

 

[rocketscientist628]

Yes, the hardware will be exposed to a marine environment per Mil-Std-810. I'm considering sealing the fastener using possible combinations of sleeves and inhibiting sealants, however, I would prefer a different solution. I would prefer going to another type of fastener, but haven't found much with regard to that. Like I said before, cadmium coated screws are hard to come by. I've also found out about a possibility that zinc coated screws might work, but I have no idea as of yet as to how difficult they would be to obtain and what strength issues may come into play. Thanks to all for your continued help!

 

[israelkk]

You can find CRES A-286 fastenrs at

http://supply.gsfc.nasa.gov/fasteners/Default.htm

 

[rocketscientist628]

Can someone tell me what the galvanic difference is between the stainless and aluminum 6061-T651? I'm not sure what grade the stainless is, as I've been unable to find that also, but the screw is NAS1352C02-5. This replaced MS16997 which was a cadmium plated screw, however, I don't know exactly what material THIS screw is.

 

[RikuY]

My company manufactures all aluminum structures with all stainless hardware that gets dunked in and out of the saltwater. We coat critical hardware that has to be removed for service or repair with white lithium grease. I think the lubrication of the threads to prevent galling and displacement of water are the only benefits for it though.

For anodic reasons, can you add a chunk of zinc with a mechanical fastener anywhere? This solved a lot of problems for us because our product became the ground for any stray current and started "releasing" aluminum from our structure.

 

[CoryPad]

You should attempt to use aluminium alloy screws. If you can't, try aluminium-friendly coatings from:

http://www.themagnigroup.com

http://www.doerkenusa.com

http://www.geomet.net

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[WKTaylor]

rocketscientist628...

My 2-cents worth...

I posted comments on a very similar topic, located on thread338-110764.

After reading the thread, ask me if I'm paranoid about Aluminum-CRES dissimilarity... and about the use of grease for corrosion protection.

Regards, Wil Taylor