An interesting Severus Alexander denarius

[284ad]

I bought this denarius of Severus Alexander from my local coin shop and initially I thought it could be a fouree given the rust like formations on the surface, especially beneath the arms of Spes on the reverse.




Anyway I did some investigating and removed a layer of this "rust" from the right hand side of the obverse portrait only to discover more silver underneath. This got me thinking; is this in fact just evidence of the low silver content of the denarius at this point?

This coin is less than 50% silver so could it be that this is actually just the other metals in the alloy oxidising?

[mix_val]

An interesting coin and in particular the reverse. 
I don't see much evidence of corrosion on the surface (ie porosity) so unless coin was struck with a rusty die, the "rust" has probably been transferred from the surroundings, perhaps other AE coins