A failure analysis of buried API 5L Grade X42 pipeline that leaked exhibited ghost pearlite in the corrosion scale. Ghost pearlite refers to the preferential corrosion of the ferrite grains surrounding pearlite grains in carbon steel. The 12.75-inch OD, 0.25-inch wall pipeline was installed in 1949 and was in natural gas service. The work scope for the pipeline failure analysis included visual examination, metallography, energy dispersive X-Ray spectroscopy (EDS), X-Ray diffraction (XRD) analysis, Vickers hardness and tensile tests to determine the mechanism that led to the perforation. This paper provides the failure analysis results and the damage mechanism. Ghost pearlite is not typically observed in external corrosion products of buried pipelines and presents a unique case to share the findings.
Pipelines are subjected to Hydrostatic testing during construction and maintenance1-2. The Hydrostatic test validates the pipeline integrity by ensuring that the pipeline can sustain the operating pressure. Hydrostatic tests can expose any anomalies present in the pipe that may result in failures at or a fraction above the operating pressure.
No failure during Hydrotest suggests that the pipeline is safe to operate up to the maximum allowable operating pressure (MAOP), while a failure identifies defects or critical flaws that would have likely failed in operation. Wall loss or crack initiation of pipelines in service can result from external corrosion. During a hydrostatic test an API 5L3 grade X42 pipeline with a wall thickness of 0.25-inch installed in 1949 in natural gas service exhibited a leak as shown in Figure 1. The leak was located adjacent to the 6 o’clock position.
A failure investigation was performed on the perforated pipeline. Results of the analyses are summarized in this paper.
The pipe section analyzed exhibited a perforation adjacent to the 6 o’clock position in an area where no external coal tar coating was present as shown in Figure 2. The dark external coating on the sample was compromised in several locations.
