I’m sure anyone who’s ever jump-started a car, replaced a car battery, or built a small 12V DIY nipple-torture device has seen this: a powdery, white and blue and maybe sometimes green substance on battery terminals. What exactly is that crap? What causes it? Can you spread it on crackers and eat it? Let’s figure this out.

First, no, don’t spread it on crackers and eat it. It’s not Gorgonzola, it’ll make you really sick. Don’t do it. Trust me. 

The corrosion is very common, and affects pretty much any car with a lead-acid battery, which is just about every 12V battery in any car. It doesn’t even take that long to form; corrosion does not just affect batteries on cars that have been sitting in weedy backyards for years. Look at the terminals on the 2014 Nissan Rogue owned by the sister of our very own editor-in-chief:

Ewww, right?

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Okay, so what is this stuff, and what causes it? Well, it varies a bit based on what terminal has the corrosion, positive or negative.

Since that picture right above is of the positive terminal, let’s start there. The greenish-bluish crap is likely copper sulfate that’s been exposed to a bit of moisture. This is formed when the copper in the terminal clamp reacts with the lead in the battery terminal, helped along by the transmission of electrical current and sulfuric acid from inside the battery, and maybe seeping out a bit from various seams.

The chemical reaction looks like this:

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Cu (s) + 2 H2SO4 (ℓ) → CuSO4 (aq) + 2 H2O (ℓ) + SO2 (g)

This copper sulfate does not conduct electricity very well, which is why battery performance will degrade (and, with that, you’ll get other electrical issues), and why it’s a good idea to clean this stuff off the terminal. Baking soda and water are good for removing the corrosion, because the baking soda will neutralize the acidic copper sulfate.

Now, if you have white powdery corrosion around the negative terminal, that’s different stuff and is there because of a different cause. The white powder is the result of a process called sulfation, and it’s usually caused by a lead-acid battery not being charged enough.

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This can happen if your car is used for mostly shorter trips, especially if you have a modern, electricity-hungry car. Modern cars tend to have a lot of ancillary electronics that draw a good bit of current, and if you mostly drive in short hops, the alternator may not be running long enough to properly recharge the battery.

As a result, sulfation happens. According to Battery University (Go Dry Cells!) this is what sulfation is:

During use, small sulfate crystals form, but these are normal and are not harmful. During prolonged charge deprivation, however, the amorphous lead sulfate converts to a stable crystalline and deposits on the negative plates. This leads to the development of large crystals that reduce the battery’s active material, which is responsible for the performance.

There’s actually two types of sulfation: reversible (soft) and permanent (hard). I bet you can guess the difference in the two types. The reversible type happens first, and can usually be reversed with a bit of overcharging to a full battery. Permanent sulfation is the result of prolonged undercharging, and there doesn’t seem to be much you can do about it.

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If it helps, this appears to be the chemical reaction between lead and sulfuric acid that causes lead sulfate:

Pb (s) + H2SO4 (aq) → PbSO4 (aq) + H2 (g)

The sulfation primarily happens on the battery’s internal plates, but leaks of sulfuric acid where the terminal (usually made of zinc and lead) exits the plastic battery housing can cause the sulfation to occur outside the battery.

If you look online for explanations about what battery terminal corrosion is caused by, you’ll likely see a lot of explanations involving the venting of hydrogen gas. While batteries certainly may outgas some hydrogen, it does not seem to be a major factor in the production of terminal corrosion.

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So, to sum up: the bluish crap around the positive terminal is most likely copper sulfate—if it’s more white and you have aluminum clamps, it could be aluminum sulfate, but either way it’s primarily caused by a reaction between the dissimilar metals of the clamp and terminal, aided by electricity and some sulfuric acid from the battery.

The corrosion around the negative terminal is most likely going to be white/grey/slightly yellowish, and is the result of undercharging the battery, causing sulfation.

Our own David Tracy happened to have one Jeep that was undercharging, and it had corrosion so bad at the negative terminal, the ground wire had to be replaced. Not surprisingly, David also had a Jeep that was overcharging, and while the negative terminal was clean, the positive one had the classic green crap:

So, those quick observations do seem to back up the theory.

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Got it? I hope that helps. Just keep those terminals clean as best you can. There’s a variety of commercial products designed to prevent battery corrosion, and they give pretty mixed results; I’d have to really try to do a test before I’d feel comfortable suggesting any particular one.

And don’t eat it on crackers.