I've just used lemon juice on some corroded torch terminals and it
worked well, still a bit discoloured but ok. I soaked then cleaned with
a tooth brush but you probably don't want to be soaking the calculator.
Smeared a little silicone grease on them to prevent a repeat but
vaseline would be ok if you don't have any.
uk.d-i-y is another good group for this kind of thing.