The crusty film that appears around your batteries in an electronic device is corrosion. While the appearance may seem foreboding, often corrosion only affects the actual batteries and the terminals within the device, both of which are usually replaceable or repairable. While some types of climates are humid and therefore more prone to causing battery corrosion, proper practices limit the amount of electronic devices damaged by a buildup of corrosion.
What It Means
Corrosion occurs when your electronic device comes in contact with water or moisture. The moisture essentially eats away at the battery, causing it to malfunction. Corrosion can also occur when a battery is punctured and leaks battery acid in the device. While mild corrosion can be removed and cleaned, battery corrosion can affect a device's performance and connectors, so it's important to catch and deal with corrosion as soon as possible.
In most cases, battery corrosion is fixable and should not affect your device's performance in the long run. Unfortunately, some corrosion damage is so extensive that it effects more than just the replaceable batteries. Some corrosion damages the battery terminals and prohibits a connection between the device and new batteries. The only way to inspect that damage and analyze your device is to remove the batteries. You need to then check the terminals and other components of the device that may have been affected by moisture or acid.
What to Do
As soon as you recognize corrosion, remove the batteries and discard immediately. After you've chucked the batteries, inspect your device for signs of corrosion. Specifically check the connections from the batteries to the device. If a crust film is left on them, mix equal parts baking soda and water to make a paste and use a wire brush to apply a small amount. The baking soda will begin to bubble in reaction to the corrosion. Wait until the mixture stops bubbling, then use a dry, soft cloth to wipe the mixture away. Then, use a cotton swab dipped in rubbing alcohol to wipe down the areas that may have gotten wet during the cleaning process, and your device will dry quickly without added damage.
While your device will likely still work after the batteries become corroded, constant and recurring corrosion could affect your device's internal components. It's best to avoid using your device near water, such as the pool or bathtub. If you live in an especially humid area and moisture cannot be prevented, remove the batteries from your device when it's not in use to prevent additional corrosion.