Molecular dynamics investigations into the hydrogen permeation mechanism of polyethylene pipeline material

To avoid hydrogen embrittlement and other hydrogen damage caused during hydrogen transportation using metal pipelines, it is feasible to transport hydrogen through nonmetallic polyethylene (PE) pipelines. However, gas permeation and leakage are more obvious for hydrogen delivery using PE pipelines than those using metal pipelines. To reveal the permeation and diffusion mechanisms of hydrogen in PE pipelines, the Grand Canonical Monte Carlo simulation and molecular dynamics are conducted. The solubility and diffusion characteristics of hydrogen in amorphous PE are investigated at temperatures of 270–310 K and pressures of 0.1–0.7 MPa. The influences of temperature and pressure on hydrogen permeation are also analyzed. The results show that the solubility, diffusion, and permeability coefficients of hydrogen in amorphous PE increase with increasing temperature, and their relationships with temperature are consistent with the Arrhenius law. Pressure has a negligible effect on the permeability coefficient of hydrogen in amorphous PE, whereas temperature appreciably affects the permeability coefficient. The diffusion of hydrogen in PE conforms to the "hopping" mechanism. The hydrogen molecule vibrates in the free volume pore for a long time and then quickly hops to the adjacent pores to complete diffusion. The hydrogen molecule continues to vibrate and hop, eventually moving further away from its initial position and permeating through PE materials.