316 fittings corrosion 1

[sunshine419]

Guys, I’ve got a little problem with some 316L steel fittings on a 316L pipe used for an auxiliary cooling water vent line. The fluid is sea water with 5ppm sodium hypochlorite and at ambient temperature and low pressure conditions (1.5 – 6 psi).

The fittings have only been in service for just 5 months. Would appreciate if you could have a quick look at the attached pictures, and your thoughts on the apparent corrosion or surface deposits would be most welcome.

 

[mrfailure]

Is the outside surface shown in the photo exposed to ambient air? Is the environment exposed to moisture on the OD? Is MIC a possiblility for this environment?

Aaron Tanzer

http://www.lehightesting.com

 

[EdStainless]

Wrong alloy, 316, especially with as welded joints will not survive in seawater.
The combination of seawater, chlorination, oxygen, and intermittent service assure that this will fail.
I am surprised that it lasted 5 mo.
What you see on the out side is corrosion product from the pitting on the inside.
You can probably stick an ice pick through the weld on this fitting.

Redesign the system. No threaded connections, use AL-6XN, any field welds need to be automatic orbital welds made with C-22 filler. No weld discoloration allowed, pickling is even better. You can put sections together with autogenous welds and then have them solution annealed.

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Plymouth Tube

 

[stanweld]

As Ed stated, you selected the wrong alloy for what appears to be a screwed fitting. 316 is highly susceptible to crevice corrosion in your environment.

 

[Hercules28]

I see that the tag wire has also rusted. Was it stainless? Is it possible that you had a small galvanic cell there?

 

[Frank5]

How does the 2.5% Mo in 316 Stainless steel metallurgically increases the ability to combat pitting corrosion against chloride ion? Which phase in the 316 is reponsible to combat pitting corrosion?

 

[EdStainless]

The Cr and Mo provide pitting resistance (and N also).
with 316 at 16% Cr and 2.0% Mo (if your lucky they will give you 2.05%) it is often less pitting resistant than 304 with 18.5% Cr and 0.10% N.
One other factor is that the welds in 316 will often have very poor pitting resistance due to Mo segregation. In 304 the welds are not much worse than the base metal. In as welded structures 304 (specially 304LN) will significantly out perform 316L.

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Plymouth Tube

 

[mrfailure]

Relative pitting resistance can be calculated using Pitting Resistance Equivalent Numbers (PREN). The most common form of PREN equation calculation is PREN = Cr + 3.3Mo + 16N; this is why the Mo in 316 makes it more resistant than 304.

Aaron Tanzer

http://www.lehightesting.com

 

[EdStainless]

Doing the math you can see that you can make 304LN with a higher PREN than 316L.
The thing to remember with PREN is that it estimates the maximum possible relative pitting resistance for a given chemistry.
I know of many ways to make material so that the actual corrosion resistance is a fraction of the optimum.


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Plymouth Tube

 

[cloa]

PREN is only an empirical measure (several different formulas) to compare stainless steel alloys for the stastical pit initiation process in chloride solutions. Despite enormous amount of research into pitting of stainless steels and some good theories, noone can actually pin down one reason why molybdenum (or any of the other elements) actually work. Its probably some combination of preferential complexation (ferric chloride is potent pitting agent) and pitting site local retardation.