Although it isn't pot metal which is basically a zink based material, the sidecovers and other aluminum parts on many bikes are die castings.
For those who don't know, a die casting uses a metal, multi-piece die which as a cavity inside it. The cavity produces a precision casting that requires very little machining making them an inexpensive way to make a complicated part.
The die is filled with aluminum under pressure thru gates that are usually quite small. These small gates are easily cut or broken off after the part is ejected from the die cavity.
The outer surface of the casting which is in direct contact with the cavities inner surface is very smooth and has quite a high degree of quality and integrity however, shortly after the molten metal is injected the small gates that fed the die solidify, usually before most of the metal in the die has.
As the metal in the die cavity that has not solidified cools, it shrinks.
Because the material in the gates are solid, additional material cannot feed into the cavity resulting in a very porous material.
Often, the high quality outer surface material is only .030-.050 thick.
This is the reason attempting to weld die castings is futile.
If this outer surface is machined away or if it is scratched or corroded the porous inner material can soak up fluids resulting in further corrosion.