Galvanic Corrosion Question 1

[AndrewTT]

I have an assembly per the attached drawing: Screw - S304, middle component - Beryllium Nickel, lower component - S316. This assembly has been exposed to an electrolyte (salt). The tip of the screw has corroded away. I believe this to be Galvanic Corrosion (GC). When I research GC everything says that the anode will be corroded and the cathode protected.

The long term fix for me is to change the middle component material but I am also looking for a short term fix.

Questions:

1A) SS is more noble than Beryllium so the middle component should corrode and not the screw, correct?

1B) Can the screw be the anode even though it is the more noble material?

2A) If I put an insulator between the screw and the middle component will the GC just move to the middle and lower components?

2B) Which component will be the anode now, the middle component or the lower component?

3) It I put a ceramic coating on the screw and lower component (cannot coat the BeNi component) can I minimize the GC? Is a coating too thin to insulate?

4) When I run a salt exposure test consisting of many of these assemblies some of the screw tips are corroded away and some look almost pristine (about 60%/40%). Any idea why the amount of corrosion is so inconsistent?

Thank you very much!

 

[dik]

What is the salt material? Some stainless materials do not hold up to chlorine materials, including salts.

Dik

 

[TugboatEng]

Perhaps the contact between the point if the screw and the BeNi component is creating a crevice to initiate crevice corrosion. That would explain the haphazard distribution while a true galvanic reaction would be more uniformly distributed.

 

[AndrewTT]

Thanks for the replies.

The assemblies are exposed to a salt spray (fog) as specified by ASTM B117. This is a salt solution that consists of 20% by weight of common salt (sodium chloride) dissolved in deionized water.

The parts all passed a 500 Hr. Boiling MgCl2 test. By passed I mean they exhibited no stress-cracks.

 

[SwinnyGG]

Galvanic corrosion will be uniform across the whole surface of a part, or nearly so.

What you're describing sounds like corrosion due to the crevice created between the two parts.

You will never get the exact same reaction to a corrosion environment across a large number of assemblies- but the next time you run a set, catalog where each assembly is located in the test chamber and look for patterns.

Remember that it is not possible for your test chamber to provide the exact same conditions at every point 100% of the time. It is very common for large test groups to show end result differences directly related to where they are located in the chamber.

 

[AndrewTT]

If I turned the assemblies upside down for the salt test should the problem lessen? Would this confirm that we are dealing with crevice corrosion?

 

[weldstan]

Turning the assemblies would probably nothing, if you have crevices that will induce crevice corrosion in 316 or 304SS and the surface is still exposed to the salt fog spray.

 

[AndrewTT]

So the geometry of where the screw interfaces with the dimple cannot be the crevice? It can only be defects in the screw tip?

My thought was turning it upside down might eliminate pooling in the pocket of the middle component, thus lessening or eliminating the corrosion. Now we are sure that it is crevice corrosion.

 

[SwinnyGG]

The way the screw interfaces with the dimple appears to be the crevice.

You described your test as conforming to ASTM B117- this is a 'fog' test. Deposition of salt is by condensation of salty vapor on the surface. Orientation of the part will not matter very much.

You need to do two things for certain- 1, start cataloging the location in the chamber of every sample in every test, and look for patterns in the corrosion result and 2- start adding screws by themselves into your test batches, as control samples. Determine if the screws themselves corrode, or if the crevice action is driving your failures.

 

[EdStainless]

Why do you assume that 304 is more noble than BeNi?
Did you actually measure some potentials?

I suspect that fluid pooling where the bolt makes contact.
304 won't cut it.

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P.E. Metallurgy, Plymouth Tube

 

[AndrewTT]

Every galvanic chart I can find shows S304 as more noble than any nickel alloy and much much more noble than Be. No, I did not measure the potential.

Can anyone make a suggestion for screw material that would improve the situation?

I am currently looking at getting screw samples of:
Ni200
Inconel
Monel
S316

Thank you.

 

[TugboatEng]

The Ni200 and Monel screws will be immune to the effects of crevice corrosion. Monel screws perform exceptionally well in chloride salts. I think the Ni200 should perform similarly but is quite a bit more expensive.

 

[AndrewTT]

Great. Thank you, Tug.

also, thank you to everyone else who provided input. This has been very helpful.

 

[EdStainless]

I would use the Monel (I assume 400) since it will more closely match the BeNi.
I also presume that as Inconel you mean alloy 625, that would work but you don't need it.

You need to look at the potential of 304 when it is active, it will be below that of BeNi.
Since the Be is in solution it does not make the alloy more reactive.
I presume that your BeNi is age hardened.

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P.E. Metallurgy, Plymouth Tube