How to protect exterior galvanized flashing?
Oh finally a picture. Thank You! Yeah, you could flash over it with galvanized, aluminum, copper or stainless. But, the problem is cutting it in. You'd want to match the cut-in on the chimney all the way up the roof slope's wall. That's to do it right.

But, plenty of guys cover the old with new to just caulk &amp; screw the new stuff to the wall. It's asking for future problems, but if maintained (re-caulked repeatedly) can last the life of the roof.