Characterization of Colonies of Stress Corrosion Cracks in Gas Pipelines of API 5L X65 Steel

Abstract

Gas pipelines are important and strategic systems for gas transportation across long distances. Thousands of kilometers of gas pipelines are installed in different regions of the world. Stress corrosion cracking of gas pipelines is an important issue due to leak and explosion risks involved. In this work, longitudinal and circumferential colonies of cracks found in a 30 years old Brazilian gas pipeline were characterized. The microstructure and mechanical properties of the API 5L X65 steel, on which the pipe was fabricated, were characterized. The toughness and tensile properties met the requirements of X65 and X70 grades. The scanning electron microscopy analysis of colonies of cracks (circumferential and longitudinal) revealed that they were transgranular stress corrosion cracks (TGSCC), caused by soil. The TGSCC occurs in near-neutral pH electrolytes, nucleating preferentially in external pits. The lifetime expected for the pipeline was 30 years, and the old asphaltic coating was damaged in several points of the line, which is also a determinant factor for the occurrence of this type of failure. The parts with colonies of cracks were identified for emergencial replacement before the complete substitution scheduled for the near future.