Bumper rusting process

Hello to all as I am new to this, and thanks for the great info so far.

I am wondering if anyone can help me with identifying how bumpers rust through the chrome? I have been told the rust comes through the backside of the bumper, but I am not sure this makes sense as the metal would then be porous.
Any thoughts?
Thanks
Hawg

 

Welcome from Naw-Fuk, this post should go into the actual message board, not the introductions.