A metallurgical failure analysis was requested on a 10 3/4 inch pipe involved in a pipeline failure. Metallurgical failure analysis included visual and photographic examination, mechanical property evaluation (yield and tensile strength), surface and core (midwall) hardness, radiographic inspection, scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) of the retained corrosion product, longitudinal and transverse metallographic examination, x-ray diffraction and chemical analysis of the subject pipeline steel.
The through-wall perforation occurred at approximately the 6:00 o’clock position with 12:00 o’clock denoted as “top” in the installed position. The pipeline in the vicinity of the through-wall perforation exhibited numerous other pits of lesser depth, i.e., less than pipe wall thickness. Energy Dispersive Spectroscopy (EDS) indicated the strong presence of chlorine on the pit surface, within corrosion product in the pits with lesser depths. Metallographic examination of the pipeline revealed a less than desirable pipe microstructure, however this was probably not the causative with respect to the rupture.
Failure of this pipeline was the result of chloride corrosion possibly assisted by hydrogen sulfide. The corrosion, which was internally initiated, was proceeded by a pitting mechanism.