316SS Welded Pipe Corrosion Protection 1

[cefstathion]

I work at a research lab and we are building electrolysis test stations. The feed is DI water on the anode inlet heated to 80C. The outlet of the cathode leg is hydrogen and unconverted DI water that flows into a 316SS welded pipe that acts as a water gas separator. Pressure can get up to 30 bar. The 316SS pipe is roughly 2" x 14" with (6) 1/2" thread-olets 3000# (hydrotested to 3000 psi). the Pipe is 2" schedule 80 seamless straight pipe with (2) 2" schedule 80 butt weld caps on either end. I am using the 1/2" NPT ports to install level sensors to allow control of the liquid level in the vessel. A concern was brought up about the potential corrosion risks at the welds of the vessel since it is in DI water service. The researchers have been following some procedure where they cap all the NPT ports, fill the vessel with CitriSurf, let it sit for an extended period and then rinse the vessel with DI water and test the pH until they are satisfied with the rinse. My understanding is that Citrisurf will only remove free iron from the surface of the 316 SS but not actually remove the chromium oxide layer (heat tint) caused from welding.

1) I talked to a few shops and they recommended doing a nitric-hydrofluoric acid pickling to remove the chromium depleted layer and get back to base material where chromium content is back to 16%. Is there a standard that tells you how you should protect 316SS welded pipe that is in DI Service? Is nitric-hydrofluoric acid pickling the standard or should i consider other methods like electropolishing?
2) Is there any concern of damaging the NPT threads if I get it electropolished? The shop told me they could protect the threads but i didn't get a ton of confidence from them that the threads wouldn't get damaged and then potentially not pass a leak test at 30 bar.
3)I have a shop that is fabricating the vessels for me. They specialize in condensate knockout pots and this is a modification of one of their off the shelf builds. Is there anything specific i should request the shop to do when they are making the welds since it will be in DI service? I don't know much about welding but they did tell me as a standard they purge with argon when they are welding.
4) Is there some sort of interval i should setup to do phased array or some other NDT testing to evaluate the quality of the welds to verify whatever passivation method i end up using is providing adequate protection?

 

[EdStainless]

I made a lot of tube and pipe that went into WFI service (water for injection).
This tube was hydrogen bright annealed, pickled, mechanically polished to 16uin, EP, then nitric passivated.
This is much higher purity than typical DI.
What are your conductivity and pH limits?

1. If you want to remove heat tint then you must pickle in Nitric/HF.
ASTM A380 is your reference.
We used 40% Nitric 5%HF at about 120F for 30 min. (this is from memory)

2.EP should remove a few ten thousandths of an inch of material.
The idea is to remove microscopic roughness, it had better have no impact on the macro dimensions.
But threads are a bad idea anyway, EP or not.
They will be a onetime use likely and then gall so badly that you will be stuck.
They also provide crevices where impurities will collect.

3. The welds need to have low residual delta ferrite.
In high purity systems our tubing had to be <0.5% and as I recall the welds were limited to 3%.
Can the whole thing be annealed in hydrogen after fabrication?
This would fix the ferrite issue.

4. Visual and penetrant inspect the welds.
And then hydro.
Other NDT is tough when it is a one off with no reference.
As small as this is you could X-ray the welds easily.

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P.E. Metallurgy, consulting work welcomed

 

[cefstathion]

Thanks so much for the fast and detailed reply. Reading through the forums I was hoping you would comment on this thread, so thank you for taking the time!

DI Water Specs:
- pH: ~6.5
- Resistivity: 18.2 M*Ohm

1. If my only concern is protecting the vessel from corrosion from DI water service (maybe I am overlooking other things i should be concerned with?), would you say Nitric/HF pickling is sufficient at providing protection? Would EP even be necessary?

2. I tried to avoid NPT fittings for the exact reason you mentioned, and I have already had some galling issues. I had a hard time finding a vessel that was 316 SS that had a smaller footprint then larger vessels. We tried out a lever float (eliminated the need for level sensors which was good) that was pressure rated and material compatible but the footprint was too big. We are trying to make the footprint small to get as many test cell stacks in a lab space for the researchers. The only level sensors I could find that seemed reliable and were pressure rated and 316 SS had 1/2" NPT fittings. I am trying to see if the vendor can weld Swagelok fittings onto the vessel but I am not sure if they will be able to accommodate / not sure if the sensor will work as well if the sensor tip isn't inserted into the vessel. Essentially, I just need something to separate the water and hydrogen, send hydrogen to the vent and water to the drain. The system runs up to 30 bar, if you have any ideas for a better water gas/separator I am all ears!

3. How would I know or verify that the welds have low residual delta ferrite? What is the potential risk/concern of not having low concentrations of ferrite? At this point in the process we don't care if the DI water gets contaminated as we are not using it for anything and just sending it to the drain.

4. X-ray could be really comprehensive and like you said, the vessels are small so it wouldn't be a large undertaking.

 

[EdStainless]

No EP isn't needed it is just there to improve cleanability.
The as pickled surface is as corrosion resistant as you will get.

We used a BN anti-seize product in order to minimize contamination.
It was in a synthetic grease base.

You can test for delta ferrite magnetically.
You can use a Magnagage or a Ferritiscope or a Sevren gage (my favorite).

https://magne-gage.com/gauges.htm

If you can find a Fischer FMP30C it would be fine.

https://ndtsupply.com/ferrite-conte...f-ferrite-meter/diverse-technologies-mf300fm/

https://severnengineering.com/ferrite-indicator/

The ferrite is much less corrosion resistant than the base metal and you will get some Fe in the system.
It may show up as 'rust' staining in odd places.
It usually doesn't actually hurt anything but it is unsightly.

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P.E. Metallurgy, consulting work welcomed

 

[cefstathion]

Thanks for the continued support, this is all extremely helpful.

I got two quotes back from two different vendors.
1) $40 per vessel to Pickle Passivate the vessel per ASTM A967 Nitric 5 followed by Electropolishing the ID to the best possible finish per ASTM B912
2) $1800 per vessel (ASTM A380) to preclean/degrease, Acid pick w/ nitric acid and HF and then passivate with nitric acid and DI water

A) The DI water that is going through this vessel is going to drain so I don't care about any contamination. I am only concerned with pitting corrosion at the welds. I am trying to make a case to management that it is prudent to treat the welds on the vessel in some way before putting it into service with DI water. It seems like there are many approaches and I am trying to balance risk with cost. Do you have any thoughts on the differences of the quotes listed above?

B) I am trying to understand the risk associated with NOT pickle/passivating the 316SS welds on the vessel I sent you. The vessel was hydrotested to 3,000 psi and is 2” Sch. 80, 316 SS. If we did not pickle/passivate, do you have any data on how long it would take for pitting corrosion to cause a failure in the weld? I am trying to make the case to management that treating the HAZ on the vessel is the right thing to do.

C)In industry, is it pretty standard practice to treat the welds of 316 SS if it is going to be in DI water service?

 

[EdStainless]

Quote 1 sounds too cheap and #2 is too high.
I think that you need to talk with both of them.
Weld line pitting is difficult to predict.
But once it starts you can't stop it, and it will end up through wall.
The fastest that I saw was a case of residual hydro water in a line.
They had leaks in less than a week.
High purity water is very corrosive.
The normal stuff in water (minerals mostly) keep it from attacking the Cu and Fe piping that we commonly use.
Remove those and water is an incredible solvent.
In most cases with high purity water the piping is well shielded during welding.
Usually with zero discoloration allowed (not even a gray haze).
And it well passivated prior to service.
Not so much as to protect the piping but rather to protect the products.
But that ends up being the same thing.


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P.E. Metallurgy, consulting work welcomed