Spectacular Pitting of Type 316L Stainless Steel Pipes before Going into Service

Pitting was observed on a 6-in (lScm) flare piping made of stainless steel ASTM A312 Gr.TP 316L, before going into service. The pits were in the HAZ and FZ boundary of the pipe seam welds. The FZ also was unifurmly corroded.. The SEM micrograph showed large inclusions in the welded area. EDS analysis indicated that inclusions were rich in AI, Si and S. The Streicher test indicated no sensitization in the welded area. The Hyue test indicated that the corrosion rate of samples from welded area was higher than the corrosion rate of samples from base metal in boiling nitric acid. This fact indicated the presence of inchlsions in the welded area. It seems that the existence of large inclusions in the welded area can be responsible for pitting. The general corrosion of the FZ was due to galvanic effect between inter-dendritic delta ferrite and the cored austenitic in the welded area which could have been prevented by proper annealing after welding. The general corrosion of this area could have triggered the pitting initiation as weld.

کلیدواژه ها:

Welding ، Austenitic Stainless Steel 316L. Flare Pipe ، Intergranular Attack