Galling of Stainless with Aluminum?
Galling of Stainless with Aluminum?
(OP)
Hi All,
I had a failure Yesterday, and becasue it's a new product I don't have any history and would like to get some concensus on whether or not this is a problem.
I have a large aluminum casting (356 T77) which has a tapped hole. Into the hole a stainless steel (316) pressure switch is mounted. As I said, I had a failure, one of the guys removed the switch to replace it. When he unthreaded it, most of the aluminum threads came with it. The unit had been assembled for two weeks.
Although these two materials are not suitable for contact becasue of galvanic corrosion, they were only in (dry)contact for 2 weeks. I doubt that was a contributor. Galling? I would suspect that more if the two metals were the same grade of stainless. Besides, there was no automated or high speed assembly to build heat. I am thinking that its either a poor tap in the aluminum, weak aluminum, or excessive torque?
I am assuming that the switch is passivated. The casting is not anodized. Anodizing would be helpful with the corrosion, but due to the size I doubt I can do it. I can't put anything on the threads that would leak into the casting (lubricants, etc). The casting is under vacuum.
Is there anything I am missing here? Should I just dismiss this as an overtorqued condition?
Thanks
Bill
I had a failure Yesterday, and becasue it's a new product I don't have any history and would like to get some concensus on whether or not this is a problem.
I have a large aluminum casting (356 T77) which has a tapped hole. Into the hole a stainless steel (316) pressure switch is mounted. As I said, I had a failure, one of the guys removed the switch to replace it. When he unthreaded it, most of the aluminum threads came with it. The unit had been assembled for two weeks.
Although these two materials are not suitable for contact becasue of galvanic corrosion, they were only in (dry)contact for 2 weeks. I doubt that was a contributor. Galling? I would suspect that more if the two metals were the same grade of stainless. Besides, there was no automated or high speed assembly to build heat. I am thinking that its either a poor tap in the aluminum, weak aluminum, or excessive torque?
I am assuming that the switch is passivated. The casting is not anodized. Anodizing would be helpful with the corrosion, but due to the size I doubt I can do it. I can't put anything on the threads that would leak into the casting (lubricants, etc). The casting is under vacuum.
Is there anything I am missing here? Should I just dismiss this as an overtorqued condition?
Thanks
Bill





RE: Galling of Stainless with Aluminum?
RE: Galling of Stainless with Aluminum?
If you learn that galling is a fundamental problem, can you use thread masking (e.g. fluoropolymer tape)? Since this is a solid, it would avoid liquid contamination.
Regards,
Cory
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
RE: Galling of Stainless with Aluminum?
Thanks
Bill
RE: Galling of Stainless with Aluminum?
The variables involved are numerous and can have large ranges. While the CalTech document is a good data point, you can't be sure of the properties of their testing. Which alloys, any coatings or surface treatments, what surface finish, what installation procedure (e.g. angular velocity)?
Use your data to move you in the right direction. If you identify that galling is the problem, research what surface treatment(s)/coating(s)/lubricant(s) you can use. In addition to fluoropolymer tape, there are Dry Film Lubricants (DFL) that could work for you.
Regards,
Cory
Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of Eng-Tips Fora.
RE: Galling of Stainless with Aluminum?
V
RE: Galling of Stainless with Aluminum?
Look down at the zinc based anti-seize for Al
http://www.jetlube.com/NewFiles/aseaz.html
This one is used around the boat yard
http://www.sealingdevices.com/loctite/c5a.html
Here is another one that has no Cu, Ni or Zn in it.
A little expensive but very good.
h
Here is Teflon based product that is used around boat. It pretty much does what is says.
http:
You can also check any large auto supply store, especially speed shops for an anti-seize compatible with Al. You normally find these as spark plug lubricants. Make sure it says OK for Aluminum.
RE: Galling of Stainless with Aluminum?
RE: Galling of Stainless with Aluminum?
All vacuum chamber applications I have seen and worked on employ helicoil inserts, as first mentioned above. Also, all aluminum components were nickel plated.