Batteries leaked or corroded in my Fenix flashlight. What should I do?
The first thing to do will be of course to remove the corroded batteries. Then wipe out all areas exposed to corrosion with a clean cloth or paper towel. Next make a thick solution of baking soda and water. Using an old toothbrush or some other kind of soft-bristled brush scrub any spots of corrosion that are found on any non-electrical components like the inside of the body tube, the outside of the head, or the shell of the tail cap (not the tail switch!). This will neutralize any residual acidity and prevent the corrosion from coming back. If any electrical components have been compromised by the corrosion there is not anything that can be done to help them. Next rinse off anything you scrubbed with clean water, making sure to avoid getting water on any of the internal electronics, especially in the head. Let the pieces dry completely.
Learn more about different flashlight batteries.