There is a big difference between surface corrosion and structural corrosion that affects the integrity of the rig. Clean off that area with a bronze wire brush followed by alcohol wipe and take a very close look at the pitted area. No need to cut the mast if pitting is rather superficial and shallow. You can assume that pitting on the outside of the mast is matched by the pitting on the inside of the mast. I would apply a coat of cosmoline to the bottom part of the mast, including the inside part when you get a chance to unstep the mast. Bottom part of the mast is subject primarily to compression loads so slight corrosion is not a big deal.