I'm looking for help in straightening out a controversy about wire.

Tin-coated copper wire is required by UL for certain boat applications. They don't allow the use of all-copper wire because the tin-coating protects against corrosion.

A funny thing though, copper is more corrosion-resistant that tin (according to tables of Standard Electrical Potential and Galvanic properties.)

Countering that, however, it seems that copper oxides are terrible at conducting electricity while tin oxides do it pretty well.

In short, while the tin-coating may be oxidizing faster than the copper, it maintains the electrical conductance through its skin better. In other words, you can't cut and re-splice an old oxidized copper wire while you can a tin-coated one.

Am I right so far?

I'd really appreciate links to where I can find accurate info on the conductivity or resistance of copper oxides and tin oxides.

Thanks

 

I'm not sure, it could be that the tin is sacrificial or that the tin oxide prevents the copper from being effected. If you can find the specification you might be able to get a free copy if you can show it's for school.

 

I tend to agree with mrmeval that the tin is sacrificed to protect the copper. I remember reading about it a long time ago, but I can't recall where and when. I think the term was 'sink metal' or something like that. But then again, I could be wrong...

 

Thanks but the sacrificial thing doesn't seem right.

Can anyone steer me to a place where I can find conductivity or electrical resistance for copper and zinc oxides? The only figures I've been able to find on the Internet are for the base metals.

Thanks!

 

Another thing you should be aware of is that tinning of copper wire does not mean only tin is applied. It is actually a solder tinning son up until recently (RoHS times) there was lead as well. Solder doesn't oxidise as quick as tin or copper with the lead in it.

Also is some instances when a wire is terminated the galvanic reaction between the metals at the join has to be considered. If a bare copper wire is crimped (generally in a tinned crimp terminal) will have a greater likelyhood of corrosion than a tinned wire is crimped in the same type of crimp.

 

A funny thing though, copper is more corrosion-resistant that tin (according to tables of Standard Electrical Potential and Galvanic properties.)

Actually, corrosion resistance cannot be determined by those tables. They list only galvanic activity - they predict which of two metals will corrode in sea water. Tin coating may well be 0.65 in comparison to copper's 0.35, but in air tin is more corrosion resistant than copper. Copper alloys are made more corrosion resistant by the addition of tin.

Certainly it would be bad to connect tinned and non-tinned copper where exposed to marine environment.

 

I tend to agree with the sacrificial concept. When I was in the Navy we were required to "tin", or solder coat, every electrical splice or termination point to make a longer lasting connection.

In addition to this, a ship's hull is a large, ferrous conductor and the sea water acts as an electrolite. When you put the two together you get a lot of advanced corrossion in the form of electrolosis between them. To counter this we use a cothodic (ship being the cathode) protection system. We would place sacrificial zinc anodes strategically around the ship's hull below the water line. This would balance the reaction between the ship's hull and the sea water, and slow down the corrossion.

It's been a long time since I've studied this, but that's the basic concept. Keeping that in mind, it's easy for me to rationalize that the reason to use tinned copper is protect it, by allowing the tin to be sacrificed to save the copper.

Hope it helps.

 

Tin oxide stays attached to the wire, forming a waterproof barrier which prevents further oxidation. Copper oxides, on the other hand, form loose powders (usually green) which fall off easily and leave the bare metal unprotected.
Aluminium, chrome and other metals have that passivation layer property as well.
Regards!