Objective Classified as a high-strength low-alloy steel, pipeline steel is suitable for large-scale, long-distance pipeline transportation of liquid ammonia. However, in complex storage and transportation environments, impurities such as air and water in liquid ammonia pose a significant risk of stress corrosion cracking (SCC) to pipeline steel, severely threatening the safety of pipeline operation. Therefore, it is crucial to explore the patterns of stress corrosion due to liquid ammonia under these complex transportation conditions to ensure the safety of liquid ammonia pipeline storage and transportation.

Methods Corrosion experiments and tensile experiments at slow strain rates were conducted using liquid ammonia with different impurities, to identify the SCC behaviors of pipeline steel. Based on results from morphological characterization and hardness testing for fractures, a quantitative analysis examined the stress corrosion cracking of pipeline steel due to liquid ammonia, in terms of corrosion rates, degradation of mechanical properties, SCC susceptibility indexes, and evolutionary features of micro-morphologies.

Results The pure liquid ammonia environment exhibited the lowest corrosion rate and stress corrosion cracking tendency in pipeline steel, with a stress corrosion cracking (SCC) susceptibility index recorded at 75.914%. As water content increased, the SCC tendency rose gently, while the SCC susceptibility index decreased slightly. The introduction of oxygen led to the formation of hard and brittle corrosion products on the pipeline surface, which reduced the SCC susceptibility index, increased hardness at the fractures of the pipeline steel, and induced cleavage planes and secondary cracks at the fractures. Consequently, the fracture behavior gradually shifted toward a tendency for brittle fractures.

Conclusion In the design and operation of liquid ammonia pipelines, it is crucial to strictly control the content of impurities, particularly oxygen and water. Establishing a lower limit for the SCC susceptibility index is essential to minimize the risk of SCC in liquid ammonia environments, thereby ensuring the intrinsic safety of liquid ammonia pipeline operations.