Pitting of 316L stainless steel in flare piping of a petrochemical plant

Pitting was observed on a 6-in. (15 cm) flare piping made of stainless steel ASTM A312 Gr. Tp 316L, prior going into service in a petrochemical plant. The pits were in the heat-affected zone (HAZ) and fusion zone (FZ) boundary of the pipe seam welds. The FZ was also uniformly corroded. The SEM photomicrographs showed large inclusions in the welded area, while EDS analysis indicated that the inclusions were rich in Al, Si, and S. The Streicher test indicated no sensitization in the welded area. The Huey test confirmed that the corrosion rate of samples from welded area were higher that of samples from base metal in a boiling nitric acid test.The results indicated the presence of a high level of inclusions in the welded area. Pitting initiation in HAZ and FZ may be attributed to existence of large inclusions in the welded area. The general corrosion of the FZ can be attributed to the galvanic effect between inter-dendrite delta ferrite and the cored austenitic in the welded area which could be prevented by proper annealing after welding. It is plausible to claim that the general corrosion of these areas could trigger the pitting initiation as well.