  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) elastic
interactions between phases, especially, the interactions between the oxide
film and the hydride, iv) elastic interactions between grains, and v)
interactions between hydrogen solutes and grain boundaries. In this study, we
report a multiphase-field model that incorporates all these requirements, and
conduct a comprehensive study of hydrogen-induced spot corrosion on the uranium
metal surface, including the investigation of the oxide film, multi-spot
corrosion, grain orientation, and grain boundary in the monocrystal, bicrystal,
and polycrystal systems. The results indicate that the oxide film can inhibit
the growth of hydrides and plays a crucial role in determining the correct
morphology of the hydride at the triple junction of phases. The elastic
interaction between multiple corrosion spots causes the merging of corrosion
spots and promotes the growth of hydrides. The introduction of grain
orientations and grain boundaries results in a variety of intriguing
intracrystalline and intergranular hydride morphologies. The model presented
here is generally applicable to the hydrogen-induced multi-spot corrosion on
any rare metal surface.
