Water diffusion in stainless steel 1

[SiggiN]

Hi all!

I'm in the process of selecting a stainless steel material for subsea use, the material will act as a housing for electroning components.

This equipment shall be able to stay submerged for 15years.

Wrt. preventing water diffusing into the equipment, what are the key parameters when selecting a material with low diffusion rates?

e.g. I imagine that selecting an ausentic alloy is beneficial due to a FCC structure (atomic packing = 0.74), is this a good start? are there other parameters that are more critical?

Thank you!

Regards
siggi

 

[EdStainless]

There will be no water diffusion through any SS at the temp and pressure that you are dealing with.
And austenitic alloy will be easier for form and weld.
These alloys are available in a wide range of corrosion resistance.
How deep will you be working? Deeper is a bit easier because it is colder, but on the negative side there is less oxygen.
I would select as very high corrosion resistance alloy (6%Mo super austenitic) if it was my project.

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[kingnero]

Ed, Why is less oxygen a negative thing here? Is the protective CrxOy layer less stable/unable to regenerate?

 

Hmmm, there are welders who will tell you that a flame preheating torch is 'drawing the moisture out of the steel'.

My approach is to pretend I didn't hear.

"Everyone is entitled to their own opinions, but they are not entitled to their own facts."

 

[TugboatEng]

That is exactly the problem, kingnero. That problem will be exacerbated under bolt heads and other overlap areas such as flanges or under barnacles. Stainless steel is really an awful material choice for your application. You can go with fancy super alloys as Ed suggested or go back to basics and use a copper alloy such as aluminum bronze or copper-nickel.

 

[EdStainless]

Yes, low oxygen makes re-passivation more difficult. And under deposits you will get more severe oxygen depletion.


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[arunmrao]

316 SS will be suitable for your needs.

 

[TugboatEng]

316 SS may be suitable with an extensive list of considerations. 6Mo or copper alloys will be suitable with the given information.

 

[EdStainless]

The traditional optimum performance material would be Monel.
The only downside to Cu based alloys (bronze or CuNi) is that there are some polluted waters that can cause accelerated corrosion of them.
You have to remember that standard corrosion resistance is not what you are interested in. How welds and mechanical fasteners will hold up long term with marine growth will require a better alloy that we would normally select.

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[TugboatEng]

As a box sitting at the bottom of the ocean you don't get to take advantage of the scouring effects of rapid flow. This stagnant application greatly complicates the use of stainless steels. A simple spec of dirt, heck even coatings such as paint, can be all it takes to initiate the formation of a pit.

 

[EdStainless]

Yes Tug is correct, It depends greatly on the alloy used and how it is used.
I did a little looking and the housings for the repeaters on subsea phone lines are usually either Monel or BeCu.

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P.E. Metallurgy, consulting work welcomed

 

[kingnero]

Tug, Ed,
thanks for the clarification.

 

[SiggiN]

Thank you guys!