how to deal with corrosion

Stainless has to be subjected to high moisture and/or no air movement in order to rust. If you have the tonearm in use in a normal indoor invironment then there should be no problem. Go to an auto parts store and buy a can of metal polish wadding and polish off the rust with that. Please be sure to keep the tonearm away from the cartridge, turntable, etc. while polishing. It would be best to remove the tonearm from the plinth. Make sure that all of the polish is thoroughly removed when you are finished. Do not use any kind of oil or wax on the tonearm. It will end up on your cartridge and on your records.

Methinks that your arm is not stainless steel or the 'rust' spot cam from something else dripping on it or something. At any rate, just use vinegar to clean off the spot. If the arm is stainless steel, then no protection is necessary for normal use - just don't spill any HCl or similar acids on it.
Bob P.

You also might check out something called Navel Jelly which is available at hardware stores. I've been told it is effective fot what you are trying to do. I've never used the stuff so I suggest reading the useage info on the package and maybe asking at the hardware store per their advice.

Flitz is an excellent polish for stainless. Use as above. Apply very small amount on cotton cloth or polishing pad.