Soak your Lincoln is some mineral oil or olive oil. Watch it for any signs of color change (daily, not every 10 minutes). After about a week, take it out and gently pick at the corrosion spot with a rose thorn. If nothing comes off, and your coin has not started to change color, put it back in the oil, and let it soak some more. It may take months for the oil to soften whatever is on your coin - if it does it at all. Don't use anything harder than the thorn on your coin.
When your are done with the experiment, don't try to wash the oil off the coin. Pat it dry with a soft cloth, and put it away. The coating of oil left on the coin will help protect it from future corrosion. Old time copper collectors would sometimes oil their coins with a horsehair brush to preserve them.
DISCLAIMER: The above advice is for informational purposes. Use at your own risk! I have had good success with olive oil on coins, but have found that it will lighten brass tokens if they are left soaking for months. (Olive oil is slightly acidic, according to what I have read - especially the lower quality brands.) It may not be a bad idea to check any coins on which you have left a coating of olive oil periodically, just to be safe.