Insulated copper wire turned into gray powder

[Majorana]

TL;DR Summary

Insulated copper wires (bare copper/not plated) inexplicably turned into gray powder. Wires are intact between affected spots.

Summary: Insulated copper wires (not plated) inexplicably turned into gray powder. Wires are intact between affected spots.

This is a piece of common red/black loudspeaker cable, with PVC insulation, pulled from service after about 18 months from installation. It was used to feed a small loudspeaker in an office, to provide low-volume background music during working hours. The loudspeaker began to malfunction and eventually ceased working. After checking all the connections, the amplifier, and the loudspeaker itself, the owner remover the cable and found what is shown in the photos. The two outer segments of the cables, that were located out of the floor (they went from the floor to a junction box in the wall on one side, and from the floor to the loudspeaker on the other side), are absolutely normal. The central section of cable, which ran in a plastic flexible tubing for electrical cables under the floor (about 3.5 meters/11 feet) shows something inexplicable. The black wire is ok, only the red wire is affected. The red insulation shows, at irregular intervals, several slight "swellings". Diameter in correspondence of the "swelling" is 0.5 to 1 millimeter larger than diameter of unaffected sections, and the length of each swelling is about 8-10 millimeters. If I cut longitudinally into a swollen section, I discover that the copper wires (bare copper, not tin/silver plated) have disappeared, converted into a grey powder (the powder had been blown away before taking the photos). I haven't seen anything like that before, nor I have read or heard of anything of the same sort. I wish to understand the reason why only the red-insulated wire was affected, and not the black. I wish to understand the reason why only some short spots were "pulverized", while the copper wire between the spots is intact! The comment of the office's owner: "It was pure luck that it was just a stupid loudspeaker cable, but if it was something else? A power cable? A grounding cable? A security system cable?". But above all, my thought is: if a brand-new cable, installed in an office, could deteriorate to that condition, apparently for no reason, then ANY cable, installed anywhere (especially outdoors) might end up the same way, without knowing the reason why! I cannot accept that such a thing could happen without explanation. Remember, it was a brand-new cable when it was installed, and it took only 18 months to get to that condition. And only in the section which ran under the floor (the floor was not new, it was built several decades ago). Needless to say, the office is a full-indoor city office, no exposure to weather.
If you need further details, please ask.
To the moderator(s): in case this is not the best forum for the topic, please feel free to move it wherever it belongs.
Thank you.

 

[Guineafowl]

I’d guess they were copper-coated aluminium, and something in the red PVC coating reacted with the aluminium. Perhaps there were some defects/pinholes in the copper coating. I’d also guess they were cheap cables.

 

[bhusebye]

Just because you installed it a certain time ago does not mean it was not manufactured a long time ago. Oxidation can happen on copper exposed to high humidity easily. Unless you were there the day it was made you have no idea when it rolled off the production line, probably came over on a shipping container from China like everything else.

 

[Guineafowl]

Just because you installed it a certain time ago does not mean it was not manufactured a long time ago. Oxidation can happen on copper exposed to high humidity easily. Unless you were there the day it was made you have no idea when it rolled off the production line, probably came over on a shipping container from China like everything else.

Two things:

1. If were oxidised copper, I’d expect green/black.
2. I’d expect the corrosion to be worst at the ends and work its way in, if it were age/humidity.

 

[Averagesupernova]

I've seen aluminum wire do that. I would suspect it is not copper and is aluminum.

 

[Majorana]

Dear all,
thank you for your answers.
Unfortunately, none of the above seems to have hit the target so far.
I have to specify that:
- although no production date is printed on the cable, it was purchased from a widely known store that normally sells a great deal of such cable to car stereo installers and hobbysts, so it's unlikely that the cable got old while in the store...
- the material of the conductors is BARE COPPER. My technical formation is in industrial electronics, I can tell the difference between aluminium, plated copper, and bare copper, please... ;-) Anyway, here is a picture of one end of the cable, with the PVC insulation stripped...
- when the cable was installed one and a half year ago, the exceeding length was cut away and stored in a drawer, in the same office. I recovered it, and I can tell that it does NOT show any anomaly (just like the two extreme portions of the installed cable that, as I explained above, didn't go under the floor but went straight up from the floor to the junction box on the wall on one side, and to the loudspeaker on the other side).
Thank you again :0)

 

[Guineafowl]

I can tell the difference between aluminium, plated copper, and bare copper, please...

What about aluminium coated with copper? That’s what we think’s involved, and your picture doesn’t show a scrape of the wires. Very common in cheap imported electronics, Al wire with Cu coat can also be identified by it’s tendency to spring back when bent.

 

[Majorana]

What about aluminium coated with copper? That’s what we think’s involved, and your picture doesn’t show a scrape of the wires. Very common in cheap imported electronics, Al wire with Cu coat can also be identified by it’s tendency to spring back when bent.

My lens is not good enough to magnify the cross section of the cut wires, but please trust me when I tell you that it's PURE COPPER, not copper-coated aluminium. It's my job, I think I know my trade... thank you.

 

[NascentOxygen]

Corrosion due to trapped vapour is going to be centered at the lowest-coldest points in the cable where the vapour is densest or condenses, at other points the warmer or cyclic room temperatures tend to keep vapour moving. A DC component in the cable current may aggravate electrochemical corrosion, and I believe some cheap radios do allow DC through the speaker coil.

 

[Baluncore]

The problem is chemical damage, and only in the red cable.

My guess is that an impurity in the red PVC insulation batch, (recycled?), was not properly mixed and was included in the cable during the extrusion process. Alternatively the wire was wet and/or contaminated prior to cladding.

Swelling indicates the copper has been converted to a higher volume Cu compound. Maybe the reaction progressed at higher temperatures while the cable was being shipped, in storage, or in the duct.

Hydrous copper chlorides can be a white to greenish or blue grey.
https://en.wikipedia.org/wiki/Copper(I)_chloride

 

[Majorana]

I think that @NascentOxygen and @Baluncore are really on something here. We may have an explanation for the two outer segments being intact. And the swelling could be just due to the fact that the final compound has a higher volume (i.e. lower density) than pure copper. Ok, I buy it. But, if I understood right, the conditions that must be present at the same time to cause such a strange deterioration in just a few months are:

1) Some kind of impurity included only in the red PVC insulation
2) Presence of a DC component superimposed on the AC audio signal, causing some galvanic attack

At this point I feel that any insulated cable carrying a DC current is at risk, since it would need just one factor (the impurity) to trigger catastrophic degradation, the other key factor (DC) being already present.
Do you think that a chemical analysis of the gray(ish) powder could help in understanding the nature of the contaminant?

@NascentOxygen , when you say "trapped vapour" do you mean WATER vapour?

 

[Baluncore]

2) Presence of a DC component superimposed on the AC audio signal, causing some galvanic attack

I think it is most unlikely that DC current was involved. It takes a differential voltage to drive an electrochemical reaction, but there is only one conductor involved. Any reaction was initially short circuited by the copper conductor. By the time the wire continuity was broken, the damage had been done.

 

[Borek]

Grey powder doesn't look like anything copper related. Your guesses as to what kind of manufacturing fault could end with a cable was not entirely copper are as good as ours.

Can you try to cut open some longer section of the cable that was not installed under the floor and check how the wires look like?

 

[anorlunda]

Is it possible that the powder was more greenish than greyish? (apologies to the English language.)

 

[Guineafowl]

Grey powder doesn't look like anything copper related. Your guesses as to what kind of manufacturing fault could end with a cable was not entirely copper are as good as ours.

Can you try to cut open some longer section of the cable that was not installed under the floor and check how the wires look like?

Despite the OP’s protestations, I’m sticking with the copper-coated aluminium wire idea. You don’t need a microscope, just a scrape with a knife and the copper coat will come off, leaving a shiny silver-coloured core.

 

[Klystron]

The floor is several decades old. Raised flooring used for cable runs may also have been used as part of an air-conditioning plenum introducing water vapor and/or cool air under the floor.

Even with a separate plenum and air ducts, cold air descends into under floor space in an air-conditioned building. Flexible plastic conduit may exacerbate this effect permitting condensation over the 18 month period possibly during wet or hot weather.

 

[Majorana]

Dear all,
thank you so much for all your contributions.

Going backwards:

@Klystron the floor is not a raised floor with plenum, ducts etc. It's an ordinary, old concrete floor, with plastic tubing for electrical wires embedded in it.

@Guineafowl I'm really unable to see your point in sticking with the aluminium idea, despite all the contrary evidence. Until now, I did not want to mention the fact that I am an experienced electronics designer, that I also designed electronics for military aircraft in service nowadays, and that I have handled, stripped and soldered electrical wires of any sort for almost 50 years now, but now I feel obliged. Any scientist knows that trying to shape evidences in order to match a theory leads to faulty results. I know very well that an aluminium wire would explain this phenomenon much easily than a copper wire, but unfortunately this is not the case. I might borrow a camera with macro lens just to post a picture of the cross-section of the copper strands, but I feel that this thing has gone too far.

@anorlunda yes, it's perfectly possible, and in addition the colour of that powder was not perfectly homogeneous. The next step I wish to take would be to have that powder analyzed. It's not so easy to find a lab willing to do the job for a private citizen. In case I find one, I would certainly post the results here, be sure of that.

@Borek as I mentioned a couple of times in my above posts, the two external sections of the cable that did not lie under the floor (they went vertically from the floor to a junction box on the wall on one side, and to the loudspeaker on the other side) are completely normal and unaffected, as well as a length of excess cable left over after installation and stored in a drawer in the same office.

 

[Guineafowl]

@Guineafowl I'm really unable to see your point in sticking with the aluminium idea, despite all the contrary evidence.

Please don’t take my Al wire idea as a slight on your credentials, rather as just a hypothesis that I haven’t quite seen enough evidence to ditch.

I feel that this thing has gone too far.

A feature of forum chat where strangers type things at each other without the nuance of body language or voice. I bet if we were sitting in a pub garden with the wire sample between us, one of us would have applied pen knife to wire to settle the matter, and we would have moved on much quicker.

In short - given your experience, it probably is pure copper; if you have the sample handy, just scrape it, take a picture or just tell us what you see. No macro lenses, just a knife.

 

[Majorana]

if you have the sample handy, just scrape it, take a picture or just tell us what you see. No macro lenses, just a knife.

No need to scratch. I cut the wire with scissors. Just looking at the cut exposed section is enough: it's all copper-pink.

 

[Guineafowl]

No need to scratch. I cut the wire with scissors. Just looking at the cut exposed section is enough: it's all copper-pink.

Ok. Looking forward to the analysis of the powder!

 

[Borek]

as I mentioned a couple of times in my above posts, the two external sections of the cable that did not lie under the floor (they went vertically from the floor to a junction box on the wall on one side, and to the loudspeaker on the other side) are completely normal and unaffected, as well as a length of excess cable left over after installation and stored in a drawer in the same office.

Yes, that's what you wrote earlier - but to me it is ambiguous whether you judged the cables just by inspecting them from the outside and seeing they didn't swell, or by cutting the isolation open to see what's inside. If they corroded only in some points, perhaps they are not homogeneous at their length, with some parts being pure copper and some not.

 

[Majorana]

Yes, that's what you wrote earlier - but to me it is ambiguous whether you judged the cables just by inspecting them from the outside and seeing they didn't swell, or by cutting the isolation open to see what's inside. If they corroded only in some points, perhaps they are not homogeneous at their length, with some parts being pure copper and some not.

Okay, it is theoretically possible that a length of wire is copper while another segment is/contains another metal, but I feel it's quite a remote possibility. Nevertheless, "impossible" and "unlikely" are entirely different concepts. At this point, I believe that the best action to take is to have the grayish powder analyzed. As I said before, while there are plenty of clinical analisys labs out there, finding a general chemistry (inorganic) analisys lab available to a private citizen is not so straightforward, at least where I live. I'm on the hunt anyway.

 

[Baluncore]

Okay, it is theoretically possible that a length of wire is copper while another segment is/contains another metal, but I feel it's quite a remote possibility.

The manufacturing process requires each fine wire be drawn at speed through a die. I would expect any change in the metal composition to damage the wire during that process.

Individual wires are then spun to form the cable. If there were patches of different composition in the individual fine wires, it would not be possible to align them all so as to react and swell in the same place in the multi-strand cable.

I believe contamination of the spun wire bundle, or the insulation must be the prime suspect.

 

[Majorana]

The manufacturing process requires each fine wire be drawn at speed through a die. I would expect any change in the metal composition to damage the wire during that process.

Individual wires are then spun to form the cable. If there were patches of different composition in the individual fine wires, it would not be possible to align them all so as to react and swell in the same place in the multi-strand cable.

I believe contamination of the spun wire bundle, or the insulation must be the prime suspect.

Okay, but even if the individual wires contained an extraneous metal, or some kind of pollutant (possibly from the die plate), this mere fact cannot - in my opinion - trigger such a catastrophic corrosion, almost a chain reaction if you allow me the term, and especially if you consider that ONLY the length of cable that lied under the floor was affected, not the two vertical sections on the wall!

 

[anorlunda]

and especially if you consider that ONLY the length of cable that lied under the floor was affected, not the two vertical sections on the wall!

Regardless of the composition, it is hard to visualize a mechanism that fits those facts.

A flaw in a section of wire that coincidentally gets installed under the floor?

A sub floor environment so radically different? Even submerged speaker cables shouldn't rot like that.

A galvanic action that applies only to the sub-floor section?

A bio-chemical theory sounds too far-fetched to imagine.

I'm stumped.

 

[Majorana]

I'm stumped.

That makes two of us!

 

[NascentOxygen]

My hypothesis is that something corrosive got splashed onto the roll of copper wire before it was plastic-coated, and moisture trapped in the cable condensing on the cooler lower level exacerbated the corrosion. The corroded patches seem roughly equi-spaced and of uniform size. I wonder is it possible for you to remove the insulation from that length of cable that was fixed to the wall, looking for signs of a corrosive substance present on the copper, but which has not proceeded to a catastrophic conversion into powder?

 

[anorlunda]

My hypothesis is that something corrosive got splashed onto the roll of copper wire before it was plastic-coated, and moisture trapped in the cable condensing on the cooler lower level exacerbated the corrosion.

That sounds reasonable. However if we uncritically accept the facts as presented in this thread, exactly the damaged length of wire (no more, no less) was installed under the floor and not in the walls. The double coincidence is too unlikely.

The other approach is reflected in most of the posts in this thread. That is to critically reject the facts given as being incorrect or incomplete in some material way.

 

[Klystron]

The common simplest element to this thread is position. The damaged section of black speaker cable was placed under the floor. What was the room used for during the 18 month period?

Speakers imply human use. Likely a break room, reception, and/or work area.
This usage implies food, drink possibly shop materials =~= liquid spills.
Coffee, tea, dressings and many solvents are acidic. Rooms are regularly cleaned and painted.
The red cable operates under different electrical conditions from the black.

Inexpensive plastic conduit may have pinholes, cracks or slits that permit seepage.

[Edit: corrected cable insulation color black to red.]

 

[Majorana]

@anorlunda, and don't forget that only the red-insulated wire was affected, the black is intact!
@NascentOxygen the corroded patches are uniform in size, but NOT equally spaced. You may have got that impression from the short segments depicted, but it was not possible to take a photo of the whole cable length (the "swellings" would be almost invisible)
@Klystron the room is just a small reception with a desk, a window (closed most of the time) and two doors. One door faces a small waiting room, the other door gives entance to the shop. The customers come in from the waiting room and talk to the receptionist who sits behind the desk. I am not aware of any "regular" spillage of any sort. Remember that the length of cable under the floor (all affected by corrosion on the red wire) is about 3.5 meters, and the floor is not of the floating type, but just a concrete floor entirely covered in ceramic tiles (a few cc of beverage spilled on it could hardly reach the cement under the tiles).

The black cable operates under different electrical conditions from the red

What do you mean with "different electrical conditions"? The circuit is a plain series circuit, amplifier's output on one side and a 4-ohm loudspeaker on the other side: it means same current in both wires, and same voltage (if any) from them to the ground.

 

[NascentOxygen]

However if we uncritically accept the facts as presented in this thread, exactly the damaged length of wire (no more, no less) was installed under the floor and not in the walls.

I don't believe that to be true, simply based on it being too coincidental. I suggest that the cable was in pretty much undamaged condition when installed, and that copper corrosion largely occurred subsequent to installation. Visual examination of the copper core in the red cable over the length where bulging is not evident should confirm/deny my hypothesis.

 

[Majorana]

I beg you pardon for NOT posting more photos, I have some problem with my camera (have not yet fully learned how to use the d@mn thing!) but will try to post a "picture in words". I cut open (longitudinally) three sections of red cable between swollen spots, and the copper strands are undamaged: corrosion - or whatever kind of degenerative process it is - only and neatly takes places in those 8-10mm spots. The second thing I discovered was ONE, and only one, corrosion spot on the BLACK-insulated cable. It was very difficult to see because the swelling, that is already small on the red cable (slightly more than 0.5 mm in diameter increase) was even smaller on the black cable, almost not noticeable, so it escaped detection until now. Important note: that single swelling on the black cable was exactly in correspondence with a swelling on the red cable.

 

[hutchphd]

Is the "black" output terminal from the amplifier held at local ground potential? This might explain the differential color response to a ground leakage (apologies if this has been considered previously).

 

[sysprog]

Some anaerobic bacteria can cause pure copper to corrode to copper sulfide, which is grey to black in color, and outgassing from the reactions combined with sulfide bond breaches of the insulator could account for the bulging. It could be that a section of a coil of wire was exposed to seepage of something that spottily penetrated the insulation at irregular intervals the length of which was partially dependent on the vagaries of contaminant distribution, and partially on the diminishing circumference of the coil as it was unwound.

 

[Majorana]

P.S.: the corrosion spot on the black cable went unnoticed until yesterday not only because of the less prominent "swelling", but also because there is no discoloration of the black PVC, whereas the red PVC is noticeably discolored where swollen.
@hutchphd the amplifier's negative output is insulated from the chassis, but I cannot say whether the two are internally connected, and hence grounded through the yellow/green wire in the power cord. But consider that the PVC insulation of the speaker cable is quite thick and was undamaged (the cable was new from the store when installed) until we cut it open to see inside, and the cable was housed in a plastic tubing embedded in this old concrete floor, so an electrical contact of some sort of the wires with the ground seems, if not impossible, at least quite difficult.