Filiform corrosion aka "filamentary corrosion" or "underfilm corrosion".

I politely beg to differ with the dire prognosis suggested previously, as I've generally found filiform corrosion to be readily checkable with an acid bath, I prefer phosphoric. though I suppose oxalic would work. In my experience filiform has been superficial. I'm not saying it can't erupt from within the tube, but I haven't seen that often. Mostly it's sweat spawned.