Battery leaks was an issue in the 80s and 90s, and since quality and innovation is constantly improving, I thought battery leak might be slowly going away. But I found that even nowadays, name brand alkaline batteries still leak after a while, even if originally concealed in a tight package of the metal shell. Is it true that the chemical in an alkaline battery inherently expands in volume over time, and make the alkaline battery leak issue unavoidable?
The reason for leaks in alkaline batteries is that as batteries discharge — either through usage or gradual self-discharge — the chemistry of the cells changes and some hydrogen gas is generated. This out-gassing increases pressure in the battery. Eventually, the excess pressure either ruptures the insulating seals at the end of the battery, or the outer metal canister, or both. In addition, as the battery ages, its steel outer canister may gradually corrode (in alkaline batteries there is some potassium hydroxide, a very strong base) or rust, which can further contribute to containment failure. (see http://en.wikipedia.org/wiki/Alkaline_battery#Leaks )
I also found this online :
Heat is the biggest culprit. Storing your flashlights under the seat of your car, or in direct sunlight will cause the batteries to leak. The reason they leak is because they are filled with a chemical paste of which the liquid portion is composed of a base called Potassium Hydroxide. When the batteries get hot, the paste inside expands, and forces chemicals out of the seals in the bottom and top of the battery. [...] Regardless of the brand you buy,[...] all batteries will leak eventually, so if you don't use the flashlight for extended periods of 3 months or more, remove the batteries, or change them out every couple months. (from here)