Stress Corrosion Cracking Observed in Ex-service Gas Pipelines: A Comprehensive Study

The properties affecting stress corrosion cracking (SCC) in ex-service API 5L X42 and X65 pipeline steels were investigated to determine the differences between the two grades. Field-grown sections of SCC affected pipelines were studied to determine what effect the physical properties had on SCC growth. Using electron backscatter diffraction (EBSD) and a newly developed technique, SCC affected grains were separated from unaffected grains to determine the microstructural properties influencing SCC. It was confirmed in both grades that cracks primarily propagated between high angle grain boundaries and most coincident site lattice boundaries didn’t offer increased resistance to SCC. Σ3, Σ7, and Σ11 boundaries were present in higher proportions in uncracked grain boundaries than cracked in both grades. Crack arrest locations typically had high levels of {110}//RP (rolling plane) and {111}//RP textured grains ahead of cracks whilst cracked grain boundaries tended to contain both {110}//RP and {112}//RP textured grains. Uncracked outer surfaces of pipe in both grades contained mostly {110}//RP textured grains. Transgranular cracking was observed in both grades in isolated locations and tended to occur in {100}//RP and {111}〈112〉 textures.