NACE MR0175/ISO 15156 provides the severity of a sour environment defined as a function of pH and H2S partial pressure for sulfide stress cracking. Although hydrogen-induced cracking (HIC) is also a major issue of linepipes exposed to a sour environment, the severity diagram does not necessarily correspond to the HIC susceptibility. Recently, from the viewpoint of fitness-of-purpose, the severity diagrams for HIC have been proposed based on the concept of equal hydrogen concentration. From this concept, HIC susceptibility corresponds well to the steady-state hydrogen permeability obtained from the hydrogen permeation test method. In this study, in order to clarify the relationship between HIC susceptibility and hydrogen diffusion behavior, in situ crack inspection during the HIC test and finite element method analysis of hydrogen diffusion were performed. The in situ inspection technique using phased-array ultrasonic testing which can capture the time dependence of crack distribution, and HIC initiation and propagation behaviors were clearly visualized and investigated. It was found that crack initiation and propagation stabilize as hydrogen concentration reaches a steady-state value throughout the specimen and the result was considered evidence that the HIC susceptibility corresponded with the steady value of hydrogen permeability.

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