Causes of the field flowline weld joint rust-through damage

The paper presents results of the flowline pipe analysis in order to determine the causes of the butt weld joint rust-through damage (wormhole). Using Baumann sulfur print technique, the presence of iron sulfides scale on the inner pipe surface was studied. Using X-ray diffraction technique, the phase composition of the corrosion products was determined. It was found that chemical composition and mechanical properties of the metal from the studied flowline fragment comply with standard requirements. The resulting hardness values including those in the weld joint zones indicate that the metal has a certain corrosion-cracking resistance. Results of metallographic studies of longitudinal polished samples with full product thickness show that the residues of the weld capping pass remained on the bottom of the corrosion pit in the area of the observed rust-through hole. They have multiple pores up to 3 mm diameter. The corrosion pit surface is covered with iron sulfide layer 1–3 mm thick. The sulfide layer thickness in the area of the corrosion pit is 10+ times higher than on the rest pipe surface; this indicate that the corrosion process progressed faster here. The authors concluded that the cause for the weld joint rust-through damage was the pit corrosion that occurred under the impact of H2S-containing fluids on the lower generating line of the pipeline in the area of the weld startstop, where the weld root side suckback was observed. Probably, there was a flaw in the first weld pass within the corrosion pit area (shrinkage cavity, incomplete fusion, pore, or other), and the accelerated corrosion was the consequence of H2S-containing liquid slug here. This suggests that there was a flaw in the first weld pass within the area of the corrosion pit that has propagated along the first weld pass start-stop line.