Liquid ammonia stress corrosion behavior of pipeline steel under water and oxygen impurities

Purpose Pipeline steel is a high-strength low-alloy steel capable of meeting the demands of large-scale, long-distance pipeline transportation of liquid ammonia. However, the storage and transportation environment of liquid ammonia is complex. When impurities such as air and water mix with liquid ammonia, the stress corrosion problem of pipeline steel becomes particularly prominent. Therefore, exploring the stress corrosion behavior of liquid ammonia under complex transportation conditions is crucial for ensuring the safety of pipeline storage and transportation.

Methods To investigate the stress corrosion behavior of pipeline steel in liquid ammonia, corrosion tests under different impurity conditions and slow strain rate tensile tests were conducted. By combining fracture morphology characterization and hardness testing, a quantitative analysis was performed on the stress corrosion of pipeline steel in liquid ammonia. This analysis considered corrosion rate, mechanical performance degradation, stress corrosion sensitivity index, and the evolution characteristics of micro-morphology.

Results The corrosion rate and stress corrosion tendency of pipeline steel were minimal in a pure liquid ammonia environment, with a stress corrosion sensitivity index of 75.914％. As the water content increased, the stress corrosion tendency slightly increased, and the stress corrosion sensitivity index slightly decreased. The introduction of oxygen impurities led to the formation of hard and brittle corrosion products, which reduced the stress corrosion sensitivity index, increased the hardness at the fracture site of the pipeline steel, and induced the fracture morphology to gradually exhibit cleavage facets and secondary cracks characteristic of brittle fractures.

Conclusion In the design and operation of liquid ammonia pipelines, it is essential to strictly control the oxygen impurity content, consider the lower limit of the stress corrosion sensitivity index, reduce the risk of liquid ammonia stress corrosion cracking, and ensure the intrinsic safety of liquid ammonia pipeline operations.