Hydrogen diffusion analyses of a cracked steel pipe under internal pressure

In hydrogen diffusion in metals, we have to consider not only the hydrogen flux driven by the gradient of hydrogen concentration but also the hydrogen flux driven by the gradient of hydrostatic stress. Such hydrogen diffusion is a multi-physics problem coupled hydrogen diffusion with a stress field. We performed such hydrogen diffusion analyses for a steel pipe with a crack on an inner surface under internal pressure. On the inner surface of the pipe, the hydrogen concentration calculated using the fugacity of the hydrogen molecules was used as a boundary condition for a hydrogen diffusion equation. The finite element method was employed both for a hydrogen diffusion analysis and for a stress analysis. We obtained the hydrogen concentration around a crack tip by varying the magnitude of internal pressure and the crack length, and examined the relationship between the maximum hydrogen concentration and a nonlinear fracture mechanics parameter CTOD (Crack Tip Opening Displacement). Good correlation is found between the maximum hydrogen concentration and CTOD.