Products that use alkaline batteries should not be left unused in extreme temperatures. The battery could leak and cause rusting, which could damage the product, be it a flashlight, camera, or even a toy car. Leaking from the battery is potassium hydroxide, which hardens as soon as it is exposed to the atmosphere. It prevents fresh batteries from powering the device and can be hazardous to your skin.
Take the batteries out of the device and dispose of them. Make sure you do not get any of the material on your skin if the leak is still in liquid form.
Mix an equal proportion of water and vinegar or water and lime juice. The idea is to get an acidic solution to clean the corrosion.
Use a cotton swab or a clean cloth to dab the solution onto the corrosion. What you use will depend on the size of the area to be cleaned but in most cases, a swab will suffice. Use a toothbrush on those surfaces that are a bit bigger and have stubborn corrosion stains after applying the solution. Be careful of excess solution running into a battery-operated device---you could damage the device.
Use a blow dryer set on cool to dry out smaller spaces or a clean cloth to wipe out any moisture. Never insert new batteries unless the product is completely dry.
Run water over your skin for about five minutes to completely cleanse your skin if it has come in contact with the rust. Potassium hydroxide can be extremely harmful to the eyes and can burn the skin.
Check the brand of the alkaline battery. Some bigger companies offer service for such leaks or offer advice for dealing with a particularly bad leak.
If you install batteries incorrectly, they may leak. Make sure to install the batteries according to the grid in the battery compartment. Supervise children when they install batteries to make sure they install them correctly.