Причины сквозного повреждения сварного соединения трубопровода

Abstract

The article presents the study results of the flowline pipeline to determine the causes of the through hole (weld hole) in the butting side weld of pipelines. It was made assessment by the Bauman method of sulfur imprints for the presence of deposits of iron sulfides on the inner surface of the pipes. The phase composition of corrosion products was determined by X-ray diffraction analysis. It was revealed that the chemical composition and mechanical properties of the metal of the studied fragment of the pipeline comply with regulatory requirements. The hardness values, including those in the weld zones, obtained from the test results, indicate that the metal is resistant to corrosion cracking. According to the results of metallographic studies on longitudinal full-thickness thin sections, it was established that in the through hole area at the bottom of the corrosion spot, there are remains of the facing seam layer, where there are multiple pores with a diameter of up to 3 mm. The surface of the spot is covered with a layer of iron sulfide with a thickness of 1–3 mm. The thickness of the sulfide layer in the spot is ten or more times greater than on the rest of the pipe surface – this shows that the corrosion processes developed here faster. The cause of the damage to the welded connection of drill line is pit corrosion, which occurred under the influence of H2S on the lower part of welded lock, where was observed concavity of the joint root. Probably, in the area of the corrosion pit, there was a defect in the first layer of the weld (suck-up, faulty penetration, pore or others, etc.), and accelerated corrosion was the result of the accumulation of H2S fluid here.