00:14 Oct-12-2006 Nigel Engineering, - Specialist services United Kingdom, Joined Oct 2000 1094
Looking for root erosion in thin-walled duplex stainless steel welds
We have air coolers on our plant (oil & gas) part of which consists of a row of 1 inch dia, 1,6 mm to 3,5 mm wall thickness duplex (22% Cr, UNS31803) tubes TIG welded with matching filler metal to an 8 inch duplex header (wt 11 - 18mm). Weld ferrite content range 35% - 65%.
I am no corrosion engineer but it seems that duplex is susceptible to weld root erosion or cracking in case of wet-upset in an H2S environment. So we wish to check the weld roots for such whilst new equipment fabricated from material more-suited to the potential environment are procured.
A simulation test rig of one header with 4 or 5 tubes welded in place is being made for performance demonstration purposes. Do anybody have experience of such configurations? Is low-frequency eddy current a possibility? Or DC MPI - knowing that the permeability variations may produce non-relevant indications. As the material/configuration set-up is difficult to inspect I expect signal interpretation and evaluation will be a problem. Any ideas?
Re: Looking for root erosion in thin-walled duplex stainless steel welds This type of corrosion known as intergranular corrosion. It result due to depletion of chromium to be concentrated on grain boundaries during welding of high chromium stailess steel. Traces of nitrogen within range (0.1 - 0.5%) added as alloying element in duplex stainless steel can prevent this type of corrosion. The mechanism occured by catching chromium as chromium nitrid which prevent depletion then corrosion cracking.