Diffusion can be solved in Mechanical APDL - please see the "Coupled Field Analysis Guide" under the Mechanical APDL Help documentation. There are different types of coupling available, such as structural-diffusion, meaning that the concentration can cause 'diffusion strain'.
I don't know if in your application hydrogen concentration causes any noticeable diffusion strain, but I just mention that such coupling capabilities are available, but you can also solve diffusion problems directly, too. However, you could post in Structures forum as Rob mentioned for more details/help.
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