Safety Comes First
Box 4520, Milton FL 32572; E-mail: [email protected]
Black Wire Disease
In the September column I wrote a short piece regarding the storage of Ni-Cd batteries and a condition known as "Black Wire disease" that was discovered while I was charging one of my flight packs. As is normal for me, if I find a 20 percent or so reduction in useful battery capacity, I remove the shrink wrap and check the cells for one that might be failing.
On this particular occasion I discovered an odd-looking wire corrosion that made the copper look as if it had turned black along most of the length of the lead and into the connector. Not knowing the cause, I started asking people with electrical knowledge if they had seen anything similar, and perhaps knew what caused it.
While looking into potential causes, I discovered an article on the Web that described the condition and offered a few tips on how to prevent electronic damage or failure. Ian L. McQueen used the article shortly after it was published and provided a counter-opinion that included specific facts regarding why Ni-Cd batteries can't cause the problem.
Confused and wondering why this is relevant to safety? Electronic failure (although rare in this case) does happen if not detected and corrected early. If you're interested in knowing what to look for, the possible causes, and prevention, a review of Ian's letter is in order:
"I read with interest your column in the September issue, particularly the last two pages regarding Ni-Cd batteries. While the information about the cells themselves is probably reliable since (I understand) Red Schofield was involved with their manufacture during his working days, there is a serious problem in a paragraph that you quote, apparently from Red:
'The reason I recommend removing the batteries from the transmitter and airplane is to protect against Black Wire disease. If a cell shorts while in storage, there is a high probability that there will be some leakage that can ultimately lead to the black-wire problem.'
Red can be presumed correct as far as the statement that 'there will be some leakage ...' but the important fact that I wish to bring to your attention is that nickel-cadmium cells do not cause black-wire corrosion. I have been trying to get the word out on the subject of black-wire corrosion (BWC) for several years, but 'Ni-Cds cause black-wire corrosion' is almost like a section of a catechism and this belief has become dogma.
Most times my letters have been ignored in favor of the widely believed 'orthodoxy' and do not reach print so that readers could judge for themselves, while the 'Ni-Cds cause BWC' fallacy has been repeated again and again.
The reason why I state flatly that Ni-Cd cells do not cause black-wire corrosion is because they cannot. The only active chemical in Ni-Cds is potassium hydroxide (KOH), a material similar in its properties to lye (sodium hydroxide). When copper is placed into a strong KOH solution the reaction is very slow, but eventually the copper is converted to copper hydroxide. Copper hydroxide is a green-blue chemical, and it dissolves in water.
The material formed when black-wire corrosion occurs is, naturally, black. It is also insoluble in water. Stated baldly, the electrolyte in a Ni-Cd cell simply cannot cause black-wire corrosion.
There are also other factors that make the 'Ni-Cds are guilty' story unbelievable. The corrosion usually takes place in wires with the black insulation, the universal color for the negative lead. When one of these is connected directly to a cell, it is soldered to the bottom end of the case, at the far end of the cell from the seal and vent. If any wire were to be affected by the KOH, it should be the wire(s) closest to the seal/vent. This is normally not the case.
It defies common sense to believe that any electrolyte that might escape would find its way selectively to a wire at the far end of the case and 'ignore' wire immediately adjacent to the vent.
More proof: Black-wire corrosion is found in numerous cases where Ni-Cd cells are not present. I first noted the phenomenon in the early 1970s when an earphone lead became unserviceable when the copper inside the polyvinyl chloride (PVC) insulation crumbled into black powder.
I have since then had numerous pieces of wire showing the phenomenon, including a coil of 16 feet of wire (with red insulation) that suffered from black-wire corrosion along its full length. This wire had never been used and had been hung on a nail and left for a year or two. It had never been remotely near a Ni-Cd cell.
The following are defining characteristics of black-wire corrosion:
- The corrosion is uniform along the full length of the wire, and it is found only inside PVC insulation.
- It is impossible for the blackening to be present if the wire is exposed. I have found the black powder a few millimeters inside the insulation, but it is sound beyond that point. This is in strong contrast with BWC, which occurs from one end of a wire to the other, as I mentioned before.
The only logical conclusion that can be drawn regarding the cause of BWC is that some chemical from the PVC insulation is causing the black-wire corrosion. I suspect that sulfur is used in some way in formulating the PVC, for the black material certainly fits the description of copper sulfide, which is black and insoluble in water.
Eventually I hope to have some BWC-affected wire tested using a scanning electron microscope, which can identify the elements present in a sample, but this is in the future. For the moment, the important thing is to get the word out to modelers that nickel-cadmium cells do not cause black-wire corrosion, so that they will not waste their time trying to counter a problem that does not exist.
What can they do to protect themselves from black-wire corrosion and the potential for a power failure and crash? Whether they see any sign of leakage from the cells in their transmitter and receiver battery sets or not, they should regularly check the lead wires from the batteries. It would even be wise to peek back the insulation, or cut a small nick in the insulation, to check the color of the wire inside the insulation. If the wire has turned black, replace the wiring at once."
Ian mentioned that in the past he had found wire that had turned black and had never been near a Ni-Cd battery. Having thought about it for a while, I also remember as a kid seeing rolled wire in workshops with the same type of corrosion.
Since PVC wasn't used back then, I wonder what the cause was for the condition I remember seeing. Maybe it was more green than black. Oh well, I'll keep checking my electronics at least annually to make sure they don't catch the disease!
That's all for this month. Stay warm this winter, and use plenty of ventilation when painting indoors!
MA
Transcribed from original scans by AI. Minor OCR errors may remain.
