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May 23, 2017

What is that white stuff that leaks from batteries?

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Why do batteries leak?

Carbon-zinc batteries (LeClanche cells)

When you open a dead flashlight you might find a scarry multicolored mess. White powdery gunk, and corrosion of the terminals. A carbon-zinc flashlight battery consists of a zinc can filled with manganese dioxide and liquid. The liquid is water with either ammonium chloride or zinc chloride dissolved in it. The battery leaks when the zinc can is dissolved. The liquid at this point can contain ammonium chloride and zinc chloride. There is also a starch paste used as the separator, and this can also leak out.

When the battery is totally discharged the reaction products include manganese hydroxide, zinc ammonium chloride, ammonia, zinc chloride, zinc oxide, and water. So for a carbon zinc battery the white gunk is a mixture of these compounds and starch. None of these are toxic, in fact they can all be used in fertilizer.

But WHY do they leak? The battery power comes from a chemical reaction which consumes the zinc. The zinc can eventually dissolves due to the main reactions and other side reactions, which is why it is more likely to leak when left to discharge even when it is dead, as when your five year old leaves the flashlight on and hides it under his bed.

Alkaline batteries

Alkaline batteries are very similar to carbon zinc batteries. They use manganese dioxide and metallic zinc as the reactive materials, but they use an alkaline potassium hydroxide solution for the electrolyte instead of the mildly acidic ammonium chloride. The reaction products are manganese oxide and zinc oxide with by-products of manganese hydroxide and zinc hydroxide. So the white residue consists of manganese oxide, zinc oxide, potassium hydroxide, zinc hydroxide, and manganese hydroxide. Soon after oozing out, the potassium hydroxide reacts with carbon dioxide in the air to form potassium carbonate. Again, none of these chemicals is toxic, and all can be used in fertilizer.

Some people will look up the MSDS and see that there is potassium hydroxide electrolyte and warn you that this is corrosive and could eat your skin and blind you. There isn't any free liquid, it is all absorbed into the battery components. If you cut the battery and smeared the black manganese dioxide powder on your hands you might get a slippery feeling as the lye converts oils in your skin into soap. But as mentioned above, a few hours exposure to atmospheric carbon dioxide will convert the KOH into K2CO3, which is perfectly safe. The KOH is very soluble, but the potassium carbonate isn't so much, which is a test to see if the reaction has occurred. In a flashlight the ingress of CO2 is slow, so the KOH has time to corrode the innards of the flashlight.

But WHY do alkaline batteries leak?

There are side reactions that generate hydrogen gas--basically the zinc breaks down the water. This gas will build up in pressure and eventually vent, driving some of the liquid electrolyte out with it. Manufacturers at one time uses mercury to form a non-reactive skin on the zinc to prevent this, but of course due to government mandates mercury is no longer used in alkaline batteries.

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