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BACKGROUND

I am in the process of restoring an old computer. The problem with the computer is that it has a NiCad battery fitted to the motherboard, which powers a real-time-clock circuit. The NiCad battery has leaked onto the motherboard. The leaked material has caused quite extensive damage, which I am cleaning and repairing. See here and here.

I am curious about this leaked material. Here is what I (think) I understand so far:

  • NiCad batteries use potassium hydroxide as an electrolyte, and this is the material which has leaked from the battery.
  • The potassium hydroxide forms potassium carbonate when it reacts with the carbon dioxide in the surrounding air.

MY QUESTIONS

  1. Some white(ish) powder has formed on the battery terminals themselves. Is this potassium carbonate?
  2. The effect of the battery leakage has been quite widespread corrosion of metals, mainly electronic components. The corrosion has spread much further than the leakage site. How?
  3. Why does the material attack metal? Do both materials (potassium hydroxide and potassium carbonate) attack metal, or does one of these have more of an effect on metals than the other?
  4. Are any other chemicals likely to have formed that I have not considered? (e.g. there are green/blue deposits to be found in areas affected by the leakage)

CONJECTURE

Having pieced together some information from reading on the internet, I have formed a bit of an opinion about the process which has unfolded here. And I'd like to run it by you all to see what you think about it:

  1. When the battery started to leak, the potassium hydroxide reacted with the air and formed potassium carbonate, as a white powder on the battery terminals.
  2. When all of the carbon dioxide in the air was depleted (due to closed computer cover), the potassium hydroxide stopped forming potassium carbonate. And this is when it began to attack the metals.

THANK YOU IN ADVANCE!

The purpose of my questions is purely to satisfy my curiosity. If someone takes the time to respond, and share their knowledge, then I respect that and I am grateful. So I just wanted to say thank you in advance!

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Your conjecture is at least partially correct, in that there was a mix of $\ce{KOH}$ and $\ce{K2CO3}$. However, there is only ~400 parts per million (ppm) $\ce{CO2}$ in air, so at first, the $\ce{KOH}$ was free to react with metal (and, possibly, the green lacquer on the PC board), until enough $\ce{CO2}$ entered the cabinet to form the carbonate. It is possible some raw $\ce{KOH}$ is still present, under a film of carbonate, so be careful. Note that there is ventilation in a PC cabinet, so $\ce{CO2}$ is never really depleted in it.

Far more damage, though, may be due to electrolytic corrosion. Wikipedia has a photo similar to yours, showing "Severe PCB corrosion from a leaky PCB mounted Ni-Cd battery". The liquid electrolyte creates bridges across the bare IC pins and across the PC board traces, and the continual current from the power supply and the not-quite-dead battery cause metal to be removed from one place and deposited in another.

After removing the battery, rinsing many times with distilled water, and tooth-brushing the board, you may find that surface traces need to be rebuilt (such as by soldering a bit of braid across a gap). This is likely a multilayer board, and if an internal trace is gone, it would be difficult to find and bridge the gap. Note that any such patches will affect timing, due to the impedance mismatch, so you may find that after all repairs, crashes occur with some frequency.

Best wishes for your restoration project. It's nice to know that at least one venerable computer is not going to landfill!

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  • $\begingroup$ Thanks for your answer. I am determined to rescue this old computer. I also asked this question on Reddit, and I had an interesting response about the green/blue material that can be seen in the photos. According to one Redditor, this is likely to be Green nickel hydroxide. Are you able to comment on this? Regarding the corrosion, it appears to me that you are claiming the opposite to my conjecture: that the corrosion is caused by the KOH and then the K2CO3 forms a kind of protective layer, preventing further corrosion. Is this understanding correct? $\endgroup$ – Q'' Jul 15 '16 at 20:41
  • $\begingroup$ 1. The green-blue corrosion is likely a mix of copper carbonates with some nickel carbonate near the battery. 2. The corrosion is caused primarily by the electric current, not by the alkalis. Even salt water would cause similar corrosion if the current is on. 3. The potassium carbonate might be a shell over KOH, which is still caustic to skin. It would not have prevented corrosion. $\endgroup$ – DrMoishe Pippik Jul 15 '16 at 21:19
  • $\begingroup$ Hm. I'm surprised at your suggestion that the corrosion is caused by the electric current. I imagined the corrosion was happening due to some basic reaction between the leaked material and the metal. When you say that electric current causes it, are you referring to galvanic corrosion? $\endgroup$ – Q'' Jul 16 '16 at 9:20
  • $\begingroup$ Regarding the green/blue material, I wonder if I could take a sample of it into work; we have a machine which can analyse the material and tell us what elements it contains. This would clear it up once and for all! $\endgroup$ – Q'' Jul 16 '16 at 9:21
  • $\begingroup$ 1. "electrolytic corrosion" = "galvanic corrosion" 2. yes $\endgroup$ – DrMoishe Pippik Jul 17 '16 at 3:16

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