Origination and Propagation Mechanisms of Stress Corrosion Cracking in High-Strength Pipeline Steel Girth Welds in the Coastal Environment of the Niger Delta

Stress corrosion cracking at girth weld locations in high-strength API 5L X70 and X80 pipeline steels represents an incompletely understood failure mode in the coastal Niger Delta environment, where the combination of near-neutral pH soil chemistry, cathodic protection potential fluctuations from interference sources, residual welding stresses, and hydrogen charging from sulphate-reducing bacterial activity creates a uniquely complex multi-factor cracking environment not fully captured by existing mechanistic models. This research investigates the origination and propagation mechanisms of stress corrosion cracking in girth welds of X70 and X80 pipeline steels using a multi-scale experimental and mechanistic modelling approach. Laboratory specimens replicating as-welded, post-weld heat-treated, and mechanically misaligned girth weld geometries are exposed to synthetic Niger Delta soil solutions under applied cathodic protection potential ranges typical of field operation, with constant and cyclic stress loading. Crack initiation sites are characterised by electron backscatter diffraction, atom probe tomography, and transmission electron microscopy to establish the crystallographic and hydrogen distribution factors governing initiation susceptibility in the heat-affected zone and weld metal microstructures. A modified slip-dissolution and hydrogen-enhanced localised plasticity combined model is developed to describe the crack propagation kinetics as a function of the local electrochemical environment, stress intensity, and microstructural features. The model is validated against independent crack growth rate measurements and provides a mechanistic basis for revised weld procedure and post-weld treatment specifications for Niger Delta pipeline applications. Keywords: stress corrosion cracking, pipeline steel, girth weld, Niger Delta, hydrogen embrittlement.