Stress corrosion failure of an X52 grade gas pipeline

• Coal–tar based reinforced coating and cathodic protection for X52 steel pipeline.
• Absence of coating from many locations near rupture due to high impressed current.
• High-pH corrosion occurred on the bare areas of pipe and caused corrosion pitting.
• Microscopic examination revealed intergranular cracking.
• The lowering/submerging of pipeline produced additional stresses to cause SCC.

In the present work, the failure investigation of a 45.72 cm (18 in.) diameter gas transmission pipeline (X-52 grade steel) has been described. The pipeline was shielded metal arc welded (SMAW) construction, and was welded in the form of spiral. The protective coating, applied on the pipeline, was made of glass fiber reinforced coal–tar enamel. The initial investigation showed that the failure of gas pipeline produced a big depression in the ground underneath. The produced depression was about 6.7 m deep, 4.6 m wide and 15 m long. The visual examination of the pipe revealed the absence of the protective coating from the surface of the pipeline near the ruptured area at many places from 4 o’clock to 7 o’clock positions. On closer examination, pitting was also found at different portions of the bare areas (where the coating was absent) of gas pipeline. Furthermore, the soil in which the pipeline was buried had pH of ∼9–10 that promoted corrosion on the surface of gas pipeline. The microscopic examination showed that the ruptured portions of the pipeline contained intergranular type cracks. Moreover, the stresses produced due to lowering/submerging of the pipeline seem to have contributed to this failure. The detailed study of this rupture suggested that the possible reason for this failure was stress corrosion cracking (SCC).