Dear Mr Raghvendra,
Pastes or acid solutions used for pickling stainless steels are very corrosive and could corrode any exposed surface/s upon, even if the traces of dissolved solutions are left as it is on the weld joints. If the edge of the welds contain any unattended undercuts or any unclean weld dirt’s then acid products may accumulate and cause crevice or pitting corrosion.
For a dissimilar welds involving Inconel-182 electrodes vs 410S clad base metal (11-13% Cr & Ni-1% max) the interface between the weld bead and base metal would get attacked due to :-
· Galvanic effect ( different chemistry of Inconel 182 WM & 410s base metal)
· Due to typically higher hardness in the HAZ or 410 S.S base metal
I am not sure how much is the waiting time between Pickling +Passivation and vessel PWHT. Any temperature around 300 C or more could initiate crevice in S.S. If Pickling & Passivation is a client requirement then a stage inspection (i.e. through visual inspection) before PWHT would probably assure than any form of crevice or pitting corrosions are not occurring due to corrosive actions of pickling products.
I am attaching an extract from Avesta Welding Handbook, which illustrates :Pickling and Passivation” of S.S in details.
Machine Dynamics & Component Integrity Dept.
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From: email@example.com [mailto:firstname.lastname@example.org] On Behalf Of Chaitanya Purohit
Sent: Tuesday, January 12, 2010 3:36 AM
Subject: [Norton AntiSpam] Re: [MW:4023] Linear Indications After Pickling and passivation and PWHT
There may be following possibility :
During pickling and passivation acid might me collected at the toe of the weld for longer period the material would have dissolved in acidic medium (0.5 X 0.5 mm what you mentioned). And upon washing only surface cavity would be visible.
410S does not contain enough Cr% required for Corrosion resistance and of course does not have Ni hence it falls under martensitic SS instead of Austenitic SS which are good in corrosion resistance.
Check with you material / Metallurgy people to confirm the same and if the probable cause mentioned above is the actual root cause you need to change the method of pickling and passivation to avoid the same.
2010/1/12 PEMMARAJU RAGHAVENDRA <email@example.com>
I have a typical problem with my vendor who is manufacturing Coke drums for us.
The stage of NDT including RT is such that we need to take RT before PWHT and before overlay.
One of the section ( shell to Shell) underwent the above process and cleared. After that it was overlayed by AWS class Inconel E NiCrFe3 electrodes on base plates ( SA387 Gr.11 ClassII) with 410S cladding on either side. Later the same was subjected to Pickling and passivation and went for PWHT.
Now during APWHT stage, when we are performing PT and RT, surprisingly we are getting two wagon track linear indications on either sides of weld toes from inside ( on cladding side) which are exactly the border junction of ENiCrFe3 inconel and 410S cladding of the shell plates.
It is having a depth of around 0.5mm and width also same.
Could any one explain the cause of formation of this indication at the above juncture. As per our estimation, it could have happened due to pickling and passivation. Since another shell with above processes ( except pickling and passivation ) has not got any such indications.
Thanks for your time and efforts.
Larsen & Toubro Limited
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C. K. Purohit
Sr Welding Inspector