If you deal with vintage electronics containing rechargeable nickel-cadmium (NiCd) batteries, you surely encountered the problem: when you open that cherished device you're trying to repair, inside you find a battery covered with yucky, crystalline stuff, and all around inside, wires, circuit board traces, and even electronic components are corroded beyond repair. It is unfortunately not easy to find reliable information about this subject. Some sources go as far as claiming that the problem doesn't even exist! No matter how sound the principles are on which their scientific speculations rest, those who dealt with badly corroded vintage devices know that the problem is real.

First, the facts:

• Several types of corrosion are seen. There's white crystalline stuff usually near battery terminals; there's blue or green stuff usually near power traces; there's black corrosion that can appear anywhere. The one thing you don't usually see is the kind of mildly corrosive but yucky stuff that comes out of alkaline batteries.
• These types of corrosion appear in all kinds of devices containing rechargeable battery packs. These include pocket calculators, PC motherboards with CMOS clocks powered by a rechargeable battery, electronic toys, rechargeable flashlights, and more. If it's old and contains long-dead NiCd cells, you can bet good money that you'll find corrosion inside.

So what is this mysterious corrosion?

I am no chemical engineer, mind you, but knowing what I do know about NiCd batteries, I can make some educated guesses.

First, a bit of battery physics and chemistry. The electrolyte in NiCd batteries is potassium hydroxide, or KOH. This is actually a quite corrosive substance. It usually stays safely inside the batteries but under certain circumstances it can leak. NiCd batteries are designed to permit outgassing, which usually occurs when the battery is reverse charged (a condition that often occurs in battery packs containing several cells connected in series, when they are deep discharged.) Sometimes, however, it appears that old batteries can also leak electrolyte, in the form of gaseous KOH.

The good news is that KOH readily reacts with carbon dioxide (CO₂) found in air, forming harmless potassium carbonate (2KOH + CO₂ => K₂CO₃ + H₂O). No wonder you see all that white crystalline stuff near battery terminals! This stuff can easily be removed with a small brush, and it rarely leaves behind any residue. Sometimes the crystal appears wet; as far as I know, that's ordinary water that's bound to crystalline K₂CO₃.

The bad news is that air contains very little CO₂. When there's only a limited amount of air available (such as when your calculator is safely stored inside a box or case for a long period of time) eventually all CO₂ will be exhausted. If additional KOH is leaked from the batteries, it'll do its corrosive best, attacking both metal and plastic parts inside your calculator. The result is often black corrosion, appearing for instance on solder joints; this type of corrosion is especially difficult to deal with because when it mixes with solder, it makes the contaminated solder difficult to melt or remove. Because gaseous KOH can penetrate just about anywhere, corrosion can appear in insulated wires, even inside chips that are not in hermetically sealed cases.

So much about black stuff and white stuff... what about the blue or green stuff? That stuff, believe it or not, has almost nothing to do with batteries; it has everything to do with the presence of voltage (whether from a battery or another source) in a damp environment (that's my theory anyway.) If there's a sufficient amount of moisture in the air, electrolysis can take place involving any circuit traces, wires, or leads that are connected to a voltage source. Most circuit traces are made of copper; the result of the reactions taking place here is probably copper oxide (with its characteristic greenish-blue color), and perhaps other copper compounds. This problem can be compounded by the fact that a byproduct of the reaction between the KOH electrolyte and carbon dioxide in the air is water; so the small amount of air trapped inside a calculator can get quite damp even if the machine is stored in a dry, but not well ventilated place.

Conclusion: leaving a power source inside an electronic device for very long periods of time is probably not a smart idea. If the voltage source is a dying NiCd battery that can release corrosive gases, or if the device in question is stored in a humid environment such as a damp basement or an unheated storage room, things get worse. Vintage calculators can be worth a lot of money these days, so it pays to take good care of them!