[ezsteve]

I've been making rust by electrolysis of an iron anode in a salt solution, which all works fine until I switch it off for a while (about a week). During this time something happens to the copper cable I'm using that travels all the way back up into the transformer, destroying it. What and why is this happening? I was thinking it could have been because the cathode I was using was just the negative (copper) wire dunked into the solution, but I'd rather not waste another transformer to find out whether thats the problem.

[Rabn]

You need to provide more details concerning your half-cells.

[ezsteve]

I'm not exactly sure what you want to know, but my setup goes as follows:

I have used a range of transformers from 4.5 to 6v dc, with the positive cable connected to the top of an iron anode (so that the copper cable is not submerged) in a sodium chloride solution. The negative wire is simply dunked into the solution about 3 inches from the anode. As I've said, the iron oxide is being produced fine, it's just that when I take the transformers out of the solution and unplug them, after a while the copper begins to go dull (even when I strip the wire back).

[Rabn]

I think what may be happening is that you are pushing the equilibrium fwr to the right.

Fe(s) = Fe2O3


Since you aren't using two cells separated by a salt bridge; once you turn it off, if all of the iron oxide isn't removed and the solution changed you end up pushing current the opposite way because the equilibrium is far to the right. This causes electrons to flow in the opposite direction which both oxidizes the copper wire and destroys the transformer(probably by killing a diode). This is just a quick thought, but it may be the case.  I asked about your setup because unless I can visualize/draw it and understand the mechanisms involved there really is no way for anyone to help you solve the problem. The thing is that since you don't have two half cells it isn't really a redox reaction, it is just electron transfer through the salt solution. Check the pH of your solution after you run the apparatus. It will help you and anyone helping  you understand what is happening in the solution. for instance since the only source of oxygen is water, the solution should become acidic.

[Arkcon]

I'm not exactly sure what you want to know, but my setup goes as follows:

I have used a range of transformers from 4.5 to 6v dc, with the positive cable connected to the top of an iron anode (so that the copper cable is not submerged) in a sodium chloride solution. The negative wire is simply dunked into the solution about 3 inches from the anode. As I've said, the iron oxide is being produced fine, it's just that when I take the transformers out of the solution and unplug them, after a while the copper begins to go dull (even when I strip the wire back).

Have you considered the possibility that saline solution is wicking its way up between the insulation and the copper wire and corroding the transformer while you leave it idle?  Initially you said that you left it, but in the post above, you said the transformer is dying when unpluged.  Is it dry on the shelf, or still in solution?

[Borek]

This causes electrons to flow in the opposite direction which both oxidizes the copper wire

Huh? Do you mean that copper wire oxidation depends on the current direction?

[ezsteve]

Rabn - To be honest I don't understand, I haven't yet covered electrolysis in that much detail yet, although I don't think that is what is happening, I forgot to mention that I removed the transformer from the solution and unplugged it etc, so there shouldn't be any current flowing, should there?

Arkcon - Sorry, what I meant to say was that I take the transformer out of the solution and unplug it. I normally dry the ends of the wires as well so unless the salt solution is getting into the wire casing when it's in operation it's difficult to see that that's what is causing it.

[Rabn]

This causes electrons to flow in the opposite direction which both oxidizes the copper wire

Huh? Do you mean that copper wire oxidation depends on the current direction?

No, just that if the equilibrium is pushed far to the right it would respond by moving to the left providing the energy required to speed up the oxidation of the copper wire.

[Borek]

This causes electrons to flow in the opposite direction which both oxidizes the copper wire

Huh? Do you mean that copper wire oxidation depends on the current direction?

No, just that if the equilibrium is pushed far to the right it would respond by moving to the left providing the energy required to speed up the oxidation of the copper wire.

What kind of energy? And why this energy will speed up the oxidation of copper?

That's assuming that Fe₂O₃ is able to spontaneously reduce itself to Fe... Fe₂O₃ is thermodynamically stable, electrolysis is used just to speed up oxidation process that goes on its own in the presence of water and oxygen.

[ezsteve]

Just had a thought, would placing two sticks of graphite in the circuit prevent this; between the transformer and each electrode?

[enahs]

When you say something happens to the copper wire all the way back up to the transformer, how do you know this? Can you see it? Is it an exposed copper wire? What does it look like? What damage is being done to the transformer? How is it destroying it?


I suspect there is a much simpler answer, and that the transformer might not actually be destroyed.

Also, do you know what kind of transformer it is?

[ezsteve]

Yes, you can see it clearly and in fact it's not dull as I said earlier (it's been quite a while since I looked at them) it's black (copper (II) oxide?). Of course it could be that it's gone from dull to black in the time I've had it put away.

If I keep stripping the wire back further and further it is still black (right back to the transformer) so I'm guessing that this has either continued right back into the transformer or stopped right at the exit point of the transformer (rendering it useless anyway, since there is no way to open it).

Two of the transformers are "Nokia Type: ACP-7X" phone chargers, output 3.7v, 355mA
The other is a "Motorola SPN4680B" phone charger, output 4.8v, 350mA

Basically, using the phone chargers for electrolysis in the way that I've described causes one of the wires (not sure which, I can't remember) to turn black and stop the flow of current. I don't know why this is happening or how to stop it but it would be great if I could!

[Rabn]

This causes electrons to flow in the opposite direction which both oxidizes the copper wire

Huh? Do you mean that copper wire oxidation depends on the current direction?

No, just that if the equilibrium is pushed far to the right it would respond by moving to the left providing the energy required to speed up the oxidation of the copper wire.

What kind of energy? And why this energy will speed up the oxidation of copper?

That's assuming that Fe₂O₃ is able to spontaneously reduce itself to Fe... Fe₂O₃ is thermodynamically stable, electrolysis is used just to speed up oxidation process that goes on its own in the presence of water and oxygen.

I researched this a bit and would like to apologize for my misguided assumptions, although I think I was on the right track just didn't spend enough time thinking or researching. The reason appears to be as follows:

Since water is the only source of oxygen in the system, for every molecule of Fe₂O₃ produced there are also 6 H₃O⁺ molecules produced. This will make the solution acidic. That is why the pH of the solution after running the apparatus is important. If the solution is acidic after running the reaction; the  H₃O⁺ would  either attack the iron rod, the copper wire or the Fe₂O₃.  If the H₃O⁺ attacked the rod H₂ gas would be produced as iron readily gives up electrons.  As referenced in this article, especially note section 5: http://www.aps.anl.gov/asd/me/medsi02/papers/MED026.pdf, it states that at slightly acidic pH the corrosion of copper, which this article states as black to reddish in color, by way of dissolved oxygen is greatly accellerated. Why does the whole wire get corroded if only the end is in the solution? I bet if you were to inspect the wire you would find that there is a thin layer of paper around the wire to act as an extra layer of insulator, or some other water absorbant material, it won't be a thick layer. It may even be that there is a small gap between the polymer used to coat the wire and the wire itself, this would then wick the water along the wire via capillary action.  That explains why it takes so long for the whole wire to turn black...regardless of how the water travels along the wire it needs to come to equilibrium with the solution and over time it would oxidize the wire. The energy in the solution would be provided by the decrease in pH.  It may be that the transformers fail because the connection between the circuit board and copper wire is inhibited by the corrosion of the copper wire at the solder site. This would continue after removing the wire from the solution because you can't really dry the inside of the wire.  This article suggests that making a solution with pH of around 8 may sufficiently decelerate the oxidation process.  Oxidized copper contacts are always troublesome with cell phones and other devices where the battery contacts and the contacts on the device are flat...anyone remember the old Nintendo problem of having to blow the contacts of cassette? 

[Borek]

Since water is the only source of oxygen in the system, for every molecule of Fe₂O₃ produced there are also 6 H₃O⁺ molecules produced.

Water is not the only source of oxygen - we are talking about an open system, with plenty of oxygen around. You may be at least partially right if the solution is not aerated nor agitated during experiment.

Part about wicking solution can be correct, what I don't get is I have never seen mobile charger with 'naked' wires, they are usually covered with some insulation.