<P> Most metals (with a few exceptions, such as gold) oxidize freely when exposed to air . Aluminium oxide is not an electrical conductor, but rather an electrical insulator . Consequently, the flow of electrons through the oxide layer can be greatly impeded . However, since the oxide layer is only a few nanometers thick, the added resistance is not noticeable under most conditions . When aluminum wire is terminated properly, the mechanical connection breaks the thin, brittle layer of oxide to form an excellent electrical connection . Unless this connection is loosened, there is no way for oxygen to penetrate the connection point to form further oxide . </P> <P> If inadequate torque is applied to the electrical device termination screw or if the devices are not CO / ALR rated (or at least CU / AL rated for breakers and larger equipment) this can result in an inadequate connection of the aluminum wire . Also, due to the significant difference in thermal expansion rates of older aluminum wire and steel termination screws connections can loosen over time allowing the formation of some additional oxide on the wire . However oxidation was found not to be a substantial factor in failures of aluminum wire terminations . </P> <P> Another issue is the joining of aluminum wire to copper wire . In addition to the oxidation that occurs on the surface of aluminum wires which can cause a poor connection, aluminum and copper are dissimilar metals . As a result, galvanic corrosion can occur in the presence of an electrolyte, and these connections can become unstable over time . </P> <P> Several upgrades or repairs are available for homes with older pre-1970s aluminum branch circuit wiring: </P>

When did they start using copper wiring in houses