Objective Pipeline steel, a high-strength low-alloy material, is ideal for large-scale, long-distance liquid ammonia transport. However, the pipeline girth weld—a structurally vulnerable zone—is prone to stress corrosion cracking from impurities and stress in a liquid ammonia environment, posing serious risks to pipeline safety. Therefore, investigating the stress corrosion behavior of girth welds under complex transport conditions is essential to ensure the long service life of liquid ammonia pipelines.

Methods To investigate the stress corrosion behavior of girth welds in L360 grade longitudinal submerged arc welded pipes exposed to liquid ammonia, C-ring stress corrosion tests were conducted at 50％, 75％, and 100％ of yield strength (R_e) in both pure liquid ammonia and liquid ammonia containing impurities (H₂O mass fraction of 0.1, O₂ mass fraction of 0.2, CO₂ mass fraction of 0.15, N₂ mass fraction of 0.4). A systematic study was conducted on corrosion rate, micromorphology, and corrosion products based on micromorphology characterization and corrosion product analysis.

Results The C-ring specimen of the girth weld in L360 grade longitudinal submerged arc welded pipes exhibited minimal corrosion in pure liquid ammonia. However, in liquid ammonia containing impurities, impurities and stress synergistically accelerated corrosion. Impurities such as H₂O and O₂ promoted the formation of Fe(OH)₃ and Fe₂O₃ on the weld surface. Under stress, the corrosion product layer cracked, with surface cracks increasing significantly as stress levels rose. Although applied stress did not cause significant longitudinal cracks, it heightened pitting corrosion susceptibility on the specimen’s backside. In addition, corrosion micromorphology analysis indicated that welds were more susceptible to stress corrosion than the base metal: the corrosion product layer cracked in the base metal zone at 100％ R_e, whereas cracking in the weld corrosion zone occurred at 50％ R_e.

Conclusion When L360 longitudinal submerged arc welded pipes are used to transport liquid ammonia, strict control of impurity levels, pipe material stress, and girth weld quality is essential to minimize stress corrosion cracking risk and ensure pipeline safety.