Evolution of the methane pressure in a standard 2.25Cr–1Mo steel during hydrogen attack

The material degradation process hydrogen attack has its origin in the dissolved hydrogen which reacts with the carbon of the steel to form methane inside grain boundary cavities. Hydrogen attack involves several interacting processes such as diffusion of carbon and of the metal atoms; dissolution of carbides; reaction of C with H to methane; dislocation creep and grain boundary diffusion. In this paper, a microstructural model is presented which takes into account the above-mentioned processes within the framework of a multi-component, multi-phase continuum description. The numerical model is developed for microstructures build up by a ferritic matrix and carbides such as M7C3 and M23C6. The model is applied to predict the microstructural evolution, the growth of cavities and the resulting methane pressure in standard 2.25Cr–1Mo steel during hydrogen exposure at 500°C. They show that cavity growth and methane generation are strongly coupled, thus falsifying previous decoupled approaches to hydrogen attack.