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When the surface of copper turns that greenish oxidized color, does the resistance to current flow increase or isn't affected?

For instance if the point of contact is secure and clean but bare visible copper wire has oxidized will it still conduct electricity no problem?

Like this ground wire in my car. The resistance to negative of the battery reads 0 \$ \Omega \$ as well as from both ends of the cable itself.

enter image description here

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    \$\begingroup\$ Copper (II) oxide is black, and it forms a semiconductor junction with copper- like a diode. In fact the old Simpson 260 iconic analog multimeter used copper oxide rectifiers. No copper oxide is green. \$\endgroup\$ – Spehro Pefhany Jun 10 '16 at 20:12
  • \$\begingroup\$ So what's the green patina that forms called? Thought it was an oxide of copper \$\endgroup\$ – ohmmy Jun 10 '16 at 20:16
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    \$\begingroup\$ Apparently a carbonate. I don't know how conductive it is. \$\endgroup\$ – Spehro Pefhany Jun 10 '16 at 20:21
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    \$\begingroup\$ Will a water pipe still conduct water if it's rusted on the outside? Yes. This kind of oxidation is a problem only when you have to make two surfaces meet. Even then, in many cases the oxide layer can be so thin that you can break it and reach the underlying metal by mere scratching of the surface when you put the connector in place. And with metals that passivate like aluminum, the oxide layer can be so thin that electrons move through it by tunnel effect. \$\endgroup\$ – Sredni Vashtar Jun 11 '16 at 3:25
  • \$\begingroup\$ You cant measure the resistance of such contacts using a simple multimeter, you will need a milli or micro ohmmeter and four wire measurements. \$\endgroup\$ – Uwe Jun 13 '16 at 15:14
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The oxides are non-conductive as they have a full valence bands, but if you "dig into" the wire, you'll get to metal that isn't covered with an oxide. CuO is pink, but does not complete the valence rings, so you get Cu2O after a time, which is black. The green is either from a sulfate or carbonate. You have CO floating near the engine, you'll have some green after the Cu2O reduces. If you see green near the battery, it's because the sulfur from the the battery has electro migrated up to the connector and gone it a lower energy state there. You see this when you have a "bad cell". I'm pretty sure it's Cu4SO4 with some (OH) hydrated state.

Anyway, you need to take some steel wool and clean off wires if you want to remount them to remove the oxide.. You could put them in a glass of Coke and have the phosphate reduce the copper oxides . Everything just wants to be at a lower energy level, and if you are there, you don't conduct.

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  • \$\begingroup\$ "If you see green near the battery, it's because the sulfur from the the battery has electro migrated up to the connector". Any reference for this? \$\endgroup\$ – Sredni Vashtar Jun 11 '16 at 3:21
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    \$\begingroup\$ @SredniVashtar why does a logical statement need a reference? Apply occams razor, Where else in the environment is there going to be sulphur, especially near a source of confined nasty sulphric acid. \$\endgroup\$ – placeholder Jun 11 '16 at 3:50
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    \$\begingroup\$ Well, it is confined inside the battery. I can see at least one source of sulfur in a combustion engine: the fuel. Generally, the air is full of pollutants and S02 is one of them. By Occam's principles in my mind it is easier for the sulfur to come from the air, then it is to migrate through the electrode and then choose to go to the outside surface of a ground connector. That's why I wanted to know more than "where else could it come from?" \$\endgroup\$ – Sredni Vashtar Jun 11 '16 at 3:57
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    \$\begingroup\$ elctomigration is usually smaller things than suphate ions and at smaller scales, and solids, I'd bet on some sort of wicking effect where the terminal passes through the case, or vapour from the vents in the battery. \$\endgroup\$ – Jasen Jun 11 '16 at 4:24
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    \$\begingroup\$ Lead-acid batteries, during charging, can evolve gas bubbles; those rise and break, making a small source of aerosol particles of H2SO4 and water. So, you CAN get some venting of sulfur compounds from a car battery. \$\endgroup\$ – Whit3rd Jun 11 '16 at 10:12
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When cars and trucks burn fuel they emit sulfur dioxide and other pollutants (all bad for your health). The sulfur dioxide mixes with the moisture in the air to make a very mild sulfuric acid. this mild acid reacts with the exposed copper and turns it green. This green oxide is non conductive. However the copper wire that is crimped by the terminal connection or covered by insulation are protected from this mild acid mixture and thus stays copper bright and totally conductive. I have seen gold, silver and iron turn black. Aluminum turns powdery white from this same mild acid. You could use a white light grease to seal the metal contacts from the environment and oxidation. We use to use that grease in factory environments where petroleum lubricants (high in sulfur) where used.

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  • \$\begingroup\$ I wonder why those ground wires aren't fully insulated from the factory. There's a lot of exposed copper on both ends. Do you apply that grease once the connection has been made or on the each end before making the connection? \$\endgroup\$ – ohmmy Jun 11 '16 at 21:16
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In your photo the corrosion on the outside does not really matter, and it tends to be self-limiting- once a layer forms the corrosion slows greatly.

What matters is the connection between the plated copper or brass lug and the copper wire, and that is a crimp connection. A proper crimp joint is gas-tight and will not allow corrosion to occur within the joint.

To get a reliable gas-tight crimp proper tools should be used in accordance with the manufacturer's directions. A cheap crimp tool that just mashes the lug barrel against the wire is a recipe for unreliability. Good ones are made with precision, hardened dies and ratchet so that once a crimp is started it must be completed before the tool opens.

Here is a photo (from here) of some properly crimped connectors that have been sectioned to show the wire-lug interface. As you can see it's become pretty much a solid mass:

enter image description here

If you sliced open your automotive lug you would likely see a similar wire-lug interface that is a solid mass.

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The surface oxidation does NOT affect the conductivity of the wire unless the degradation is MUCH deeper. You have proved that for yourself with your zero-ohm measurement.

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Metallic materials, in certain conditions, can develop a thin film of ceramic material for various reasons (it is usually an oxide of the underlying metal). Ceramic materials do not conduct electricity, but the surface thin film is usually confined to a few atomic layers, so it will not affect significantly the properties of the bulk metal (provided the thickness of the metal is not subnanometric).

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    \$\begingroup\$ A dielectric is not necessarily a ceramic. Perhaps you've conflated the terms? \$\endgroup\$ – placeholder Jun 11 '16 at 3:50
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    \$\begingroup\$ Ceramic?? It sounds not right... \$\endgroup\$ – soosai steven Jun 11 '16 at 6:53
  • \$\begingroup\$ Titanium diboride: electrically conductive ceramic. Ceramic is not the right term for this; you simply mean that the metal has been oxidized. (Oxidation does not necessarily imply oxygen.) \$\endgroup\$ – Oleksandr R. Jun 12 '16 at 0:53
  • \$\begingroup\$ Read more carefully: I said "oxide", not oxidation. Also, oxides are typically ceramic materials, bond by covalent atomic bonds, i. e. there are no free electrons to implement electrical conduction. By the way, you shouldn't guess I'm wrong: if you think I'm wrong, please prove I'm wrong by citing your sources. \$\endgroup\$ – Electrical Architect Jun 12 '16 at 10:15
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    \$\begingroup\$ Copper patina (verdigris) isn't an oxide, is it? It's copper carbonate hydroxide or hydrated copper chloride. Copper is still oxidized by forming these substances. Cuprous oxide is very well known as a semiconductor. Protective oxides only form on a few metals; most are permeable. And "ceramic" doesn't just mean some inorganic substance, but specifically refers to a material that has been sintered. You can easily find sources for these statements about materials chemistry; there are too many possibilities to list here. I don't mean to be combative, but I really think it's self-evident. \$\endgroup\$ – Oleksandr R. Jun 13 '16 at 12:47

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