ELI5, what causes corrosion on battery leads or connections?

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The issue that brings the question to mind is I have a flashlight that has side by side AAA batteries, and if I leave it alone with new batteries for a month or so, green/white corrosion will occur where the batteries connect and the batteries will be dead. Curious what causes this or contributes to this happening. Besides taking the batteries out, is there something I can do to prevent it from happening? It seems to happen more in some devices than others, and the quality of the batteries doesn’t seem to be the cause.

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Normally this issue only occurs with old batteries that have been left in a device for a long time, or with cheap defective batteries. If all batteries are leaking soon after putting them in this flashlight, it sounds like this particular flashlight might have an internal short circuit or a poor choice of metals in the battery contacts (resulting in galvanic corrosion), or some other bad design issue.

Batteries are built kind of like a tin soup can, except that the positive and negative terminals need to be electrically disconnected (otherwise it would short circuit), so one end (usually the negative terminal) is separated from the rest of the body by a non-conductive rubber o-ring on the inside. Over time the rubber can degrade due to heat, dry air, or cheap materials that corrode from the liquid in the battery. Also the metal case can expand due to heat generated when discharging the battery, or corrode (rust) due to exposure to salt or other metals, opening up the seal and allowing it to leak. Cheaper batteries tend to use worse materials for the o-ring, thinner metals for the case, etc.

The liquid inside the battery is potassium hydroxide, which itself is corrosive. When it contacts the air it reacts to become potassium carbonate, the crusty stuff. The green color is due to corroded copper rusting out of the metal can. The liquid can hurt your skin or eyes if it gets on you, but the dry crusty stuff isn’t too bad if you touch it, as long as you don’t eat it or let it sit on your skin for a long time.