It might be some form of metal corrosion. The change from metal to metal oxide can be accompanied by an increase in volume which can lead to some very fanciful structures.
I have seen Aluminum turn into what looks like phyllo dough when near a coastline, exposed to salt air. The edge of a 1/2 inch thick sheet was fanned out, split and flaking away.
Are you sure that wasn't iron/steel? I've seen long term degradation of the sort you describe with those, but IME aluminum tends to get pitted and chalky.