In regards to the grunge, it might be a little bit excessive in general, but I think the bigger issue is that the Normal influence on it is much too strong. I think it should mostly affect the base color and roughness. The feet in particular almost look like they've been soldered.
It also seems to be a little random, lacking in direction. I'd recommend looking at some real-world examples of similar materials. Here and here.
thx
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Here and here.