Hydrogen embrittlement can occur in line pipes under sour and high-pressure hydrogen gas environments. The absorbed hydrogen is affected by not only the environments but also plastic strain introduced into the steel. However, even though various strains applied in various paths are introduced through pipe-forming processes and pipe laying, the effects of the strain path on the hydrogen absorption behaviour under various environments is not fully understood. This study describes the relationships among the strain path, dislocation density act as a hydrogen-trapping site and hydrogen absorption behaviour in various environments, as well as the fracture toughness of line pipe steel.
Line pipes are widely used as infrastructure for the efficient transportation of oil, gas and high-pressure hydrogen. Due to their cost advantages, these line pipes are often composed of low-alloy carbon steel. Hydrogen embrittlement occurs when low-alloy carbon steel is exposed to corrosive environments such as wet H2S (i.e., sour environments) or high-pressure hydrogen gas. Fig. 1 shows the effects of tensile strength and hydrogen concentration on hydrogen embrittlement (Matsuyama, 1989). In general, hydrogen embrittlement is affected by the strength of the material, concentration of absorbed hydrogen and external stress. The concentration of absorbed hydrogen affected by environmental factors such as the H2S partial pressure and hydrogen gas pressure. The concentration of absorbed hydrogen from each type of the aforementioned environments has been measured in previous studies. Hydrogen ingression behaviour has been mapped by key environmental factors of pH and H2S partial pressure (Kushida, 1992, Inohara, 2003, Hara, 2004, Fujishiro, 2022). In a hydrogen gas environment, the concentration of absorbed hydrogen is measured at various temperatures and pressures (Asahi, 2002, Omura, 2020). To simulate these environments or introduce large amounts of hydrogen into low-alloy carbon steel, ammonium thiocyanate, which is a hydrogen-entry accelerator, as well as H2S are often used in many laboratories, and cathodic polarisation methods are often employed (Hara, 2011, Omura, 2020). Understanding the concentration of absorbed hydrogen introduced from each type of environment is critical in selecting the steel material to be used in that environment.
