Two kinds of corrosion?

[MarkLeininger]

I'm familiar with the corrosion that shows up on trailer towing connector plugs. While replacing my trailer electric brakes, I noticed something when I spliced in the magnet wires to the trailer wiring. The individual strands of copper in the trailer wire had all turned a black color. There was no powder and the wire felt smooth and clean, but it was black. I assume this is tarnish, which is like the tarnish that appears on silver kitchen utensils. It didn't seem to affect the conductivity, it made immediate contact when I touched it with voltmeter. So are there two kinds of corrosion, the powdery white crap that shows up on copper tow plugs that are exposed to the weather, and this tarnish that appears as a change in copper color to black? Seems like to completely different things going on.

[Skyco]

Most often the copper mating surfaces in connectors used in trailer applications are tin plated. The white corrosion you see is from the tin, not the copper.

The black on the copper is black copper oxide, generally caused by water getting into the wire.

[Propjob]

Get some silicon grease, clean or resplice the connectors and squirt the grease in the ends. Also mastic and tape the splice. Acetone does a pretty good job of cleaning as I recall. JIM

[ampsucker]

Pure water and air
Copper is a metal that does not react with water (H2O), but the oxygen of the air will react slowly at room temperature to form a layer of brown-black copper oxide on copper metal.


The Pourbaix diagram for copper in pure water, perchloric acid or sodium It can be seen that copper in "pure" water is more noble than hydrogen. As a result it does not corrode in oxygen free water and the corrosion rate in oxygenated water is low. hydroxide[12]It is important to note that in contrast to the oxidation of iron by wet air that the layer formed by the reaction of air with copper has a protective effect against further corrosion. On old copper roofs a green layer of copper carbonate, called verdigris or patina, can often be seen. Another notable example of this is on the Statue of Liberty.

In contact with other metals

Main article: Galvanic corrosion
Copper should not be in only mechanical contact with metals of different electropotential (for example, a copper pipe joined to an iron pipe), especially in the presence of moisture, as the completion of an electrical circuit (as through the common earth ground) will cause the juncture to act as an electrochemical cell (as is a single cell of a battery). The weak electrical currents themseves are harmless but the electrochemical reaction will cause the conversion of the iron to other compounds, eventually destroying the functionality of the union. This problem is usually solved in plumbing by separating copper pipe from iron pipe with some non-conducting segment (usually plastic or rubber).

Sulfide media

Copper metal does react with hydrogen sulfide- and sulfide-containing solutions. A series of different copper sulfides can form on the surface of the copper metal.


The Pourbaix diagram for copper in a sulfide containing aqueous medium[12]Note that the copper sulfide area of the plot is very complex due to the existence of many different sulfides, a close up is also provided to make the graph more clear. It is clear that the copper is now able to corrode even without the need for oxygen as the copper is now less noble than hydrogen. This can be observed in every day life when copper metal surfaces tarnish after exposure to air which contains sulfur compounds.


The Pourbaix diagram for copper in a sulfide containing aqueous medium[12]Ammonia media

Copper does react with oxygen-containing ammonia solutions because the ammonia forms water-soluble copper complexes. The formation of these complexes causes the corrosion to become more thermodynamically favored than the corrosion of copper in an identical solution that does not contain the ammonia.


The Pourbaix diagram for copper in 10 M ammonia solution[12]Chloride media

Copper does react with a combination of oxygen and hydrochloric acid to form a series of copper chlorides. It is interesting to note that if copper(II) chloride (green/blue) is boiled with copper metal (with little or no oxygen present) then white copper(I) chloride will be formed.

[MrJimi]

In my experience, a wire that has burnt or got toasted will also be black.

[MarkLeininger]

Pure water and air
Copper is a metal that does not react with water (H2O), but the oxygen of the air will react slowly at room temperature to form a layer of brown-black copper oxide on copper metal.
--snip--

Thanks for info. The remaining question is whether the resistance of this layer of copper oxide is a problem, or even measurable. I measured the resistance of a short piece of wire that had this corrosion on it and couldn't detect any difference from a non corroded piece. This may be a bad experiment because even holding a probe lightly against such thin layer of corrosion may be rubbing it off.

[Skyco]

Kinda vague from the uncredited copying of a portion of a Wikipedia article but the key to the copper not being corroded by water is it has to be PURE water.
Trailer wiring doesn't get bathed in pure water.
Truly pure water is also an insulator which comes as a shock (no pun intended ) to many. In the real world all sorts of contaminants are in most water, certainly in road spray/rain water

BTW if you crimped it up good with the black oxide and it works then no problem Its just trailer wiring, not some sensitive electronic sense leads.....

[Soundguy]

Ditto that.. clean it a bit.. crimp it.. and some grease and go..

soundguy