If the operating temperature limit is below as per NACE MR-01-70 then no need of passivation. Since the reactor is weld overlayed with Stabilised grade material the chances of cracking is low. Nowadays major DHDS/HYDROCRACKER licensor are not in favor of doing passivation for stabilisation grade.It is wiser to get opinion from the licensor before proceeding the same since it depends on the feed composition like H2S etc. During my previous assignment in a refinery no passivation was carried out and catalyst unloading/loading was done under inert atmosphere.

Sastry PTA SCC vs S.S 347 clad welds resistance - Yes S.S 347 (thermal stabilized), generally, may not encounter major SCC problem. Still there could be surprises with moisture / air ingress during shut down depending on the amount of residual stresses in S.S 347 clad material welds. Operating below 400 deg cent (Debasis's opinion) may not guarantee PTA SCC free for S.S 347 clad welds during shut down with normal shut down practices and opening of man-holes with air entry in the system Inert purging and downloading is quite safer from PTA SCC for S.S 347 also - as Krish had experienced in an operating refinery practice. Some refineries practice this. Others take licensor' experience to avoid PTA SCC even with thermal stabilized S.S 347 clad welded reactors. Hope this helps C.V.Srinivasan Nishi Engineers Pvt Ltd India June 3, 2009 E-mail: nishi@vsnl.com >SS 347 is a better quality SS but it is not absolutely >immune to sensitisation but it can get sensitised at some >higher temperature above 400C to 815C and much depends upon >the Nb/C ratio in your overlay and also whether your SS 347 >weld overlay subjected to post welding thermal stabilisation >HT to add resistance to PTA SCC. > >If your HDS inside skin temperature is well below 400C the >question of PTA SCC can be ruled out. > >Think beyond before taking decision >regards