Corrosion!

Good day all.

I wanted to ask a question about corrosion. I have one wire in particular leaving a cabinet that is supposed to be 0VDC. It is working, all devices are powered and healthy. I think... Due to a running plant today I can't lift wires and really do a thorough inspection, but this 0 VDC wire is the one wire in the cabinet that seems to be corroding badly around the wire ferrule and terminal block. Its like a greenish brown corrosion that looks just like you would see around the negative battery terminal in some older vehicles.

Here comes the question,,, Why does the 0 or Neg. terminal normally corrode. Also, why does the positive terminal never corrode?

Thanks to any interested.

Chlorine in the air?

In the water industry we see a lot of green copper in areas where there is bleach or CL2 gas, and other chemicals can do this too.

It is always all the wires though, so in your case I suspect there is another root cause.

is it a grounded DC system? You could have ground current flowing through that particular wire. The source of the ground current wouldn't be the +DC terminal, it could be due to imbalanced electrical loads, induction from power lines, etc. If you have ground loops it can be particularly bad and result in circulating ground current.

PLC Pie Guy said:

Here comes the question,,, Why does the 0 or Neg. terminal normally corrode. Also, why does the positive terminal never corrode?

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That has to do with the chemical processes going on under the hood in any corrosion process. These are so-called "redox reactions". Part of the process can be negatively charged electrons that are going through a conductor while positively charged ions typically go through water (or some other conductive fluid). The direction of the electrons going through the conductor (your negative terminal) is directly related to the part of the redox reaction occurring around the negative terminal.

For a somewhat better understanding: a redox reaction must not be seen as a single chemical reaction, but rather as two closely related chemical reactions that are not necessarily happening in the same location but can happen at two distinct places which are electrically connected by means of a conductor (your wire). One of the pair of reactions is called oxidation, the other is reduction. Hence the name "redox" = "RED-OX" = "REDuction OXidation".

If you want to learn more, now search the internet for redox reactions with this in mind. It is an important subject in mechanical engineering, with relations to electrical engineering as you have discovered with the corrosion occurring around your negative terminal.

I have a ton (well, several hundred pounds at least ) of retro computers. It's always the Negative terminal of the battery that is corroded the worst.

Same reason. One terminal has a shortage of electrons and will gladly take some extra on board that are freed in one part of the redox reaction. The other has a surplus of electrons, which will happily leave that terminal to go wild into the other one of the two reactions.

So the voltage difference between the two will stimulate one part of the reaction to occur only on the neg terminal, the other one tends to happen at the plus terminal.

The part of the redox reaction that tends to happen on the neg pole is the one that frees the conductor's metal atoms (let's assume it is iron) from some electrons after which they are no longer the balanced, strong and happy participants in the strong and happy iron metal structure but have instead completely lost their mojo and have no better purpose left in life other than join with some dowdy oxygen atoms from the gutter to become lousy ironoxide losers (rust).

Old telecom systems always used to earth the positive side of the dc supply for precisely this reason. Of course this might not be feasible on your equipment for all sorts of reasons.

Andybr said:

Old telecom systems always used to earth the positive side of the dc supply for precisely this reason. Of course this might not be feasible on your equipment for all sorts of reasons.

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And British cars like my old Triumphs... (a 4, 4A, and a 6). Makes getting a US radio to work a pain.

I thought it was a European thing to earth the positive terminal in general?

V0N_hydro said:

I thought it was a European thing to earth the positive terminal in general?

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I couldn't say, the Triumphs were positive grounded, while the Fiats weren't.

V0N_hydro said:

I thought it was a European thing to earth the positive terminal in general?

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Not really. The vast majority of British or European cars have been negative earth for as long as I have been driving (since the early seventies) and even before then the positive earth models were unusual.

what about in control systems and not cars? I feel like I see more NPN stuff, sourcing inputs instead of sinking, and negative switching in European systems.