Causes of Corrosion Perforation in an Elbow of Seawater Pipeline System in a Certain Offshore Oil Field

A leakage occurred in certain offshore oilfield seawater pipeline system. It was confirmed by investigation that the leakage was because of the corrosion perforation of a pipeline elbow made of Cu-Ni alloy. The failure causes of the corrosion perforation in the elbow were studied by macro analysis, physical and chemical properties testing, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and X-ray diffraction (XRD). The results show that turbulent flow resulted from the defects produced in casting process caused flow-induced corrosion, which was the prime reason for corrosion perforation of the elbow wall. The area near the weld seam with poor passivation quality became the weak spot in the whole system and was prone to corrosion damage. Consequently, it is important to perform quality inspection to newly purchased elbow to ensure quality, and the weld seam should be thoroughly passivated to ensure the formation of protective oxide film.