The cracking is most likely chloride induced stress cracking.  When it is external, the most usual source for the chlorides is the insulation.  Weld repair of major cracks can sometimes be done.  However, usually the heat of welding will cause crack growth and/or new cracks to form.  

If the cracking is extensive, it could lead to catastrophic failure of the vessel.  In general, if a vessel has chloride stress cracking, it will need to be replaced.

Thanks for your observations which concurr with my opinions.
However,the situation I found myself in with regard to this pressure vessel working at 0.7barg at temp. of 50 degrs.C was complicated by the fact that this cracking was discovered when replacing a vessel leg support, This support required the welding of reinforcement pads to vessel shell (15mm.thk) and bottom dishing (25mm.thk)
It is true that grinding operation (preparation) exacerbated micro cracks,(this confirmed by DPI. examination)
The choice to be made during shutdown period was either continue with fitting of reinforcement pads or replace section of vessel shell.
It was my opinion that without further examination it was impossible to decide extent of chloride attack and therefore size of patch.
Before carrying out welding of pads,weldability tests was carried out on shell using 316welding rod (mma)
DPI. examination of test run indicated weld metal free of cracking.
Final DPI. of welded pads (fillet welds) show welds free from cracking in welds albeit plate material around welds do show signs of scc.

What is your considered opinion of this action and could you also expand on the possibility of catastrophic failure.

Finally, this vessel wil be monitored for leakage!.
I understand that good practice is sometimes to give outside surface a good 'wash' before re-instating correct type of insulation or vapour barrier.
Do you have any information on this?

regards.