Tank floor scan for SS 316 Chemical storage tank 1

[MATEGRITY2009]

Hi anyone,

Tank has built to API 650 and will be inspecting in accordance with API 653. Tank material is Stainless steel 316. Tank has installed at Jetty in marine environment.

Now, i looking for tank floor scan to detect underside pitting corrosion, Anyone experienced with the inspection technique for SS tank floor, please advise me.


Thank for any help.

 

[Duwe6]

Scanning with a Phased Array transducer, set up to sweep from +15° to -15° from vertical with a small sector of that same 'ducer making a straight-beam scan to record the nominal thickness surrounding the pits and other defects located by the sweeping beam. This will be slow and expensive.

Scanning using a recording UT 'box' and a 'paintbrush' transducer. Paintbrush = 'ducer with a large number of tiny elements arreinged in a row, to give several inches of coverage in each swipe.

Why do you suspect pitting of 316 s/s? The normal failure mode is Chloride Stress-Corrosion Cracking - SCC.

 

[MATEGRITY2009]

Actually both pitting and CSCC are suspected.
Pitting corrosion -We found some SS 316 piping in marine environment had noted pitting corrosion approx 1-2 deep beneath surface accumulation.
How about CSCC? Can you recommend the appropriate NDE or Advance NDE to detect this problem?

As found on 316 piping

http://files.engineering.com/getfil...-436e-9484-559a21aa4964&file=surface_accu.JPG

When removed to bare metal surface

http://files.engineering.com/getfil...75f8-4e95-9db0-523de89f9db2&file=316_pits.JPG

 

[Duwe6]

CSCC is a 'standard' problem for the Nuke Power industry - they call it IGSCC [intergranular stress corrosion cracking]. The techniques developed are essentially a) 45° small UT transducer* with a highly trained and experienced operator
b) WSY 'ducer that generates 'creeping waves' that follow the ID [far side] of the material for 1-2" and find breaks in the surface, i.e. surface-breaking cracks; requires a specially trained operator.
c) Phased Array using an angled sweep that covers 45° to 70°.

My preference would be the PAUT*, and I'd start at the weld seams - CSCC starts where the stress is highest, typically at weld seams, and typically causes transverse cracking.

*at 45° the tip of the crack gives a unique signal "crack tip diffraction", and that can be used to give an accurate depth [size] of that crack. The "Cracks are Bad, we gotta fix them" philosophy is not a good choice if the cracks are inaccessible. Sometimes 'watchful waiting' is the best method. You only have a problem when a minute crack achieves some significant, >20% depth. On the other hand, if that surface is accessible, using Dye Penetrant - PT, locate all these tiny cracks and buff them out with a sanding disk, verifying the full removal by reapplying the PT developer.

 

[MATEGRITY2009]

Thank you so much Duwe6 for the huge information.