Effect of Hydrogen Blending on Fatigue Crack Growth Behavior in Natural Gas

<p>Hydrogen and natural gas blending can be considered an important energy technology in terms of utilization of the existing infrastructure, which has a significant cost impact on the deployment of hydrogen infrastructures, and the smooth transition of energy systems. In this study, fatigue crack growth tests were carried out in air, hydrogen (H₂), 20% H₂ mixed methane (CH₄) and 20% H₂ mixed natural gases. Three kinds of natural gases with different oxygen (O₂) impurity concentrations were used in order to characterize the effect of hydrogen on the fatigue crack growth behavior in natural gas in conjunction with the effect of the O₂ impurity. The pressure of the gases except for air was 1 MPa. The test material was JIS SCM435 low-alloy steel. In the H₂ and the 20% H₂ mixed CH₄, the fatigue crack growth rates were accelerated. The acceleration was less in the 20% H₂ mixed CH₄ compared to that in the H₂. It is presumed that the lower hydrogen effect was due to reduced partial H₂ pressure in the mixture. This was confirmed by the crack growth test in 0.2 MPa H₂ resulting in the same crack growth rate as in the mixture. Regarding the fatigue crack growth tests in the 20% H₂ mixed natural gases, acceleration of the fatigue crack growth rate and suppression of the acceleration due to oxygen were observed. The oxygen effect increased with the increase in the oxygen concentration and was well-evaluated by the predictive model of hydrogen-assisted crack growth in the presence of oxygen proposed by Somerday et al.</p>