Aluminum as a conductor

[JoeN]

This is more an electrical question than an electronics question, but please humor me.

I know aluminum is used as a conductor in certain circumstances and that pure aluminum is the fourth best conductor of electricity among all known elements and compounds at room temperature, and it's far cheaper than copper.

The problem is that the oxide that aluminum forms is an insulator.  This causes problems with wiring, but you still see it sometimes, and also aluminum busbars.

What confuses me is how aluminum can be used even for those purposes since it quickly forms an oxide and we all live in an oxygen atmosphere.  How are contacts created between one conductor (say a copper wire to distribute power from the busbar) and an aluminum conductor such as a busbar, so that no oxide forms in the junction of those two conductors.   Any oxide there would have serious implications for that circuit, right?  I am thinking of this as the "last micron" problem.  Aluminum is great until it hits that aluminum oxide.  How is that last micron of oxide suppressed?

[CatalinaWOW]

The secret is that last micron.  It is thin enough to mechanically break and abrade when terminals are screwed down to aluminum wires.  And then the deformation of the metals involved seals the area off from oxygen so the barrier can't form again.

This isn't perfect (nothing is).  If the screws are not aluminum (which is very frequent), and/or the wire is not aluminum there will be differences in thermal expansion, and temperature changes will tend to let oxygen in and degrade the contacts.

[Nerull]

When aluminum is used as a conductor it is common to plate it with some other metal, which prevents an oxide layer from forming. If you have good metal to metal contact, such as in a crimp, there will be no way for an oxide layer to grow between the aluminum and the metal in any case - they will essentially be cold welded. This requires starting with an oxide free surface, though.

Tin and silver plating are both common for aluminum bus bar contact areas.

[rstofer]

Most unit substations use aluminum busbars and have done so for many decades.  And they're still in service...

We use an anti-oxidant past like:

https://www.homedepot.com/p/Ideal-NOALOX-4-oz-Anti-Oxidant-Compound-30-026/202276208

The paste is also used in crimp-on terminations and sometimes on clamp connections (like circuit breakers).  I prefer to use an aluminum crimp to copper pigtail for circuit breaker connections.

https://www.gordonelectricsupply.com/index~text~6065005~path~product~part~6065005~ds~dept~process~search

There are many connection schemes that are UL approved for Cu/Al.  They all work.