[Leica] OT - Does anyone know how to open an oxidated battery compartment?

[leica at screengang.com (Didier Ludwig)]

Does anyone know a good method to open an oxidated battery compartment? The 
metal  battery cover of a cute Petri Color 35 (which I just bought from in 
the neighbourhood) should be opened with a coin, but does not move, even 
with larger coins and rather much power. I guess it's strongly oxydated. Can 
I operate with very small drops of acetic acid, or another acid? Or 
penetrating oil? Unfortunately the battery compartment is inside the body, 
so a treatment with liquids should be done very carefully. 

Thanks in advance for any advice
Didier