material selection for load bearing screw (compression)

[AndrewTT]

I have a compression screw that has a conical tip that terminates in a spherical radius. In assembly the tip of this screw sits in a very shallow dimple. The assemble must pass many tests but the two I am here to discuss are a 30-day salt fog exposure (20% by weight sodium chloride) and a 500-hour boiling MgCl2 test (for stress-cracking).

Originally this screw was designed to be made from S303. The S303 screw exhibited a crevice corrosion type failure in the 30-day salt exposure test. The same screw geometry, made from Monel 400, successfully passes this test but loses material during the MgCl2 test. An Ammonia environment may be the more appropriate stress-cracking test for a Monel component, but the assembly MUST pass the MgCl2 test. Passing criteria is no cracking, no delamination, no degradation.

I am now considering Inconel 625, Inconel 718, and Hastelloy UNS-N10276 (C-276).

I am not a materials expert so any input is appreciated. Material/screw cost is not a huge concern. Thank you very much.

 

[EdStainless]

The first choice is C276, though 625 may work ok also.
After it is manufactured make sure that it gets acid cleaned to remove may iron that may have been picked up on the surface.
This cleaning is called passivation and is usually done with nitric acid.

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P.E. Metallurgy, consulting work welcomed

 

[AndrewTT]

Thanks EdStainless.

Would passivation with Citric acid also be acceptable?

 

[EdStainless]

As I recall (but don't trust this) it must be hot citric and the times are longer.
It could work but you would need to do some research.
It is possible that you could look at using a superaustenitic (AL-6XN) or superduplex (2507) for this application. But if machining cost outweigh material costs they may not cost enough less than C276 to justify the option.

303 was doomed from the start. Not only does it start with low corrosion resistance but the free-machining additions make it even worse.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, consulting work welcomed