Corrosion Protection of Aluminum in Contact with Stainless Fasteners

[ASIT859]

Hello,

I've done some research through the forum and have found some helpful information but am looking for some more guidance, if possible. Below is a brief description of the assembly and conditions as well as a few questions.

- A356 cast pressure vessel with an A380 cast lid. The A356 vessel will be blind drilled and tapped for the fasteners to secure the lid. This is not a nut/bolt assembly and therefore cannot be isolated with sleeving and washers.
- 18-8 or 316SS fasteners will be used to secure the lid into the vessel.
- I'm looking to have both aluminum parts hard anodized.
- I'm looking to use Heli-Coil threaded inserts in the tapped holes.
- Thread size will be 1/2-13 or larger, pending completion of joint analysis.
- This is in a fairly well protected, but outdoor environment. There is a small risk that the entire vessel will be submerged in rain water. In some locations, this water could potentially contain road salt - however, in cold conditions its unlikely enough water would be present to submerge the vessel (because cold, but I suppose it still could happen. In most cases, though, this will be in a relatively dry environment.

I've found some references in other threads on this forum suggesting a thread primer could be used to reduce chance of corrosion and other posts mention that if the amount of aluminum present compared to stainless is high, then the risk of corrosion is reduced. Does anyone have any suggestions or guidance on how to proceed and what to specify? In reference to the amounts of different material - in the entire assembly there is approximately 26 times as much aluminum than stainless, by material weight. I'm not sure if this is considered a lot of aluminum in this scenario or not.

Thank you

 

[3DDave]

Two parts:

1) area is more important than mass.
2) surface condition is more important than bulk properties.

So you are really comparing a small exposed area of stainless steel covered with a film of chromium oxide to very large area aluminum oxide in terms of relative reactivity. Limiting electrical contact with thread locking compound is a help.

Note that anodizing tends to decrease fatigue resistance quite a bit.

 

[ASIT859]

3DDave,

Thank you for the response. I'll see what more I can come up with.

Question for you or anyone else based on your statement, "So you are comparing a small exposed area of stainless steel covered with a film of chromium oxide to a very large area aluminum oxide in terms of relative reactivity." Just as a conceptual question - would a 'worse' condition for corrosion be aluminum hardware in a stainless steel assembly? Just curious I'm trying to get a grip on these things.

Thanks.

 

[TugboatEng]

A nut and bolt assembly absolutely can be isolated with sleeving and washers.

The most important thing is to eliminate crevices. Make sure the bolted surfaces are flat and parallel. Flat washer may help prevent breakage of any coating. Use nylock or acorn nuts to seal the threaded end. Do not use lock washers.

Airplanes are built out of aluminum and stainless steel fasteners. Surface treatments such as Alodine will help.

A 5000 series aluminum will also perform better if it is an option.

 

[swall]

380 aluminum does not respond well to anodizing.The silicon particles disrupt anodize film formation.

 

[ASIT859]

TugboatEng,

Thanks for the response. I may not have described the situation properly but this image linked is a good representation of the bolted assembly
- Link
The image is pulled from a Nord-Lock example - so ignore the washers in the image. This is why I believe I won't be able to use sleeving or an acorn nut.

That said, thank you for the tips. I will look into the use of a Nylok product and avoid using lock washers on the treated joint surface. Also for the recommendation of Alodine - looks interesting.

Swall,
Thank you for the tip on 380 not responding well with anodizing - I will contact our coating vendor and see what comment they have on that.

Thanks.