Help! Exhaust Flange Corrosion

[fraserb]

I'm looking for some help addressing some corrosion on my exhaust flange where the muffler meets the catalytic convertor y pipe.
I am changing out the muffler as I inadvertently damaged it.

Things I've tried:

• wire wheel brush - doesn't do anything
• Screwdriver - does chip the corrosion away, but I don't know if it will work as I get down

In the pictures attached the corrosion build up is on the bottom inside of the flange where the circular/donut gasket meets up to it.
Thanks in advance!

ps. put a photo of the new muffler compared to old one if you're interested.