If it is an alloy plate screwed into an alloy fitting, immerse them in a bucket of hot water with a bit of washing up liquid in it. Oils seem pretty useless in un-jamming aluminium corrosion, but hot (as in close to boiling) water with some soap to break the surface tension seems to do the trick. You might have to soak it for a day or two with some re-heats. If you have an old saucepan it will fit then put it on the hob and keep it hot for a couple of hours before trying to unscrew it again.