A multiphase-field model for simulating the hydrogen-induced multi-spot corrosion on the surface of polycrystalline metals: Application to uranium metal

Hydrogen-induced multi-spot corrosion on the surface of polycrystalline rare metals is a complex process, which involves the interactions between phases (metal, hydride and oxide), grain orientations, grain boundaries and corrosion spots. To accurately simulate this process and comprehend the underlying physics, a theoretical method is required that includes the following mechanisms: (i) hydrogen diffusion, (ii) phase transformation, (iii) interactions between phases and grains, especially, the interactions between the oxide film and the hydride, (iv) interactions between hydrogen solutes and grain boundaries. In this study, we report a multiphase-field model that incorporates all these requirements, and apply it to the study of hydrogen-induced spot corrosion on uranium surface. The study covers the monocrystal, bicrystal and polycrystal, and focuses on the influence of oxide film, multi-spot corrosion, grain orientation and grain boundaries on the corrosion morphology and stress. The results indicate that the oxide film can inhibit the growth of hydrides and is one of the key factors to obtain the reasonable morphology of the hydride at the triple junction of phases. The elastic interaction between multiple corrosion spots promotes the growth of hydride before the corrosion points merge. Although the introduction of grain orientation and grain boundaries results in complex intracrystalline and intergranular hydride morphologies, the physical laws and mechanisms obtained on the basis of monocrystal and bicrystal cases are sufficient to explain the more complex polycrystalline cases. The model presented here is generally applicable to the hydrogen-induced multi-spot corrosion on the any rare metal surface.