Ah! Been there, done that. You have to take out your screws more often.
Lubrication is the key, get yourself some Tef-Gel and never worry about that every again.
www.whitworths.com.au/tef-gel-corrosion-eleminator-anti-seize-lubricant-10gHonestly, the exact same thing happend to me before. I've tried everything including drilling the screws out myself and the only way I found to do a clean job was to cut the mast and re-thread it. I lost about 8-10cm of mast height but I got to keep using it.
Drilling them out requires some high-end precision machinery, I've tried finding places that could do that professionally but the height of the mast made it impossible to fit in the machinery.
Also, aluminum masts are quite cheap, so you might be able to get a new one without spending that much.
Good luck
Christian