Rover owners: What to do with dried up alarm sounder?

After my alarm has started playing up again recently (every day at 2pm as the sun shines onto the car) I decided to finally take out the sounder and leave it to dry out for a couple days.

I was expecting it to be filled with water but instead it was 100% dry with the residue of water that has been in there before left behind.

The circular piece of metal which im guessing is what makes the noise is corroded around the edge and the green circuit board is the same in some places.

Ive taken a few pics if there of any help

I have tried to circle the dodgy bits