Does corroded copper wiring have a reduced current capacity?
Copper oxidizes when exposed to oxygen in the air.
This is normal.
Your wiring is still able to carry 100% of its rated current.  

The end of your question makes me believe that you're confusing voltage (related to insulation breakdown) with current (related to 'ampacity').
Even if the wiring had corroded to the degree where it's current-carrying capacity ('ampacity') was reduced, this would have no direct effect on the insulation on the wires still being able to prevent arcing &amp; short-circuits (breakdown).