Cracking in 316L Cladding

[macor]

Hello all,
During the first inspection campaign of a ten-year old MDEA Absorber, cracking was found by PT at 5 out of 12 longitudinal seam welds of the top head 316L (NACE)cladding. Nothing abnormal in the construction MDB documentation. The vessel has been running for 10 months after the inspection campaign and now it is under inspection again. Cracking was found to be only existing at the same 5 welds. No guarantee on propagation as no record was taken during the last inspection.
Base metal material is 516 Gr 60 (NACE), PWHT'd; media: Acid gas/MDEA
Other two MDEA absorbers were inspected and found to be free from cracking.

The questions are:
1) Are the cracks inherent from construction or Environmental Assisted Cracking, and which type are they?
2) Can they be kept as-is in case they are originated from manufacturing?
3) If repair is deemed necessary, is PWHT required or we can go for NBIC/API 510 alternative preheating method?

Photos are attached.

Regards,

BV Senior Inspection Engineer

 

[EdStainless]

1)If you need to know you will have to remove coupons for lab analysis.
2) How much corrosion of the underlying material can you accept?
3) If you keep your weld repairs to the cladding then why would there be any PWHT required?

I suggest that you take samples for lab work. Either operating conditions have changed or these have been there for a while and have just been overlooked. They may not be cracks, they could be intergaranular corrosion, or they could be SCC. Or they could just be pits. You don't know how deep they are.
If they go clear through then you have serious issues. You would get accelerated attack of the substrate.
And you need to know if you are thinking of repair. If it is SCC then weld repair will make it worse. If they are IGC then it is likely to accelerate as they get deeper.

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P.E. Metallurgy, Plymouth Tube

 

[macor]

Hello EdStainless:
1) it is unfeasible to remove coupons to the lab for analysis.
2) Corrosion Allowance is Zero.
3) PWHT was a service requirement. thus for repair you either have to follow ASME VIII and go for PWHT or you follow API 510 or NBIC. my question was which one to do.

That is exactly what i am looking for: to know if they are from manufacturing or service induced cracking.
They are NOT pits. They are cracks.
Back side UT shear angle beam testing assures cracking did NOT extend to the substrate.
Can they be hot cracking, Reheat cracking or Chloride SCC (although demin water is used for amine solution)

Reagrds

 

[SJones]

Why do you need PWHT for service? The 316L should not have a problem with the environment, and the temperatures in a glycol system won't be going very low. It could well be that the PWHT damaged the 316L in the first place.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/pub/8/83b/b04

All answers are personal opinions only and are in no way connected with any employer.

 

[EdStainless]

Well if aren't willing to remove samples for analysis you should plan to replace it.
Since you say that these are new cracks this is obviously an active mechanism, so you can expect it to get worse.
But if you believe that they have been there the whole time just document them and run.
So there was no inspection on the vessel when it was manufactured? If they weren't there them then they formed in service. Are they growing? Well you need to decide if your UT is reliable, and what the risk of failure is.
They could be hot tears from the welding, they could be SCC from residual stresses, they could be IGA from carbides formed during PWHT.

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P.E. Metallurgy, Plymouth Tube

 

[weldstan]

Why is it not feasible to remove a boat sample and weld overlay repair? It takes very little time.

 

[mrfailure]

Or you could replicate if you really are unable to remove sample - at least you would be able to identify if SCC is occurring (but Stan's suggestion is the more ideal approach).