Aluminium wire terminations are inherently prone to failure. Copper wire terminations are not.
Aluminium oxide vs. Copper oxide
Both copper and aluminium oxidize on contact with air. However, the surface copper oxide is soft and electrically conductive, whereas aluminium oxide which forms immediately on exposure is strongly attached, hard and electrically insulating.
At wire terminations, which invariably involve mechanical clamping under pressure, the soft copper oxide which is itself conductive, is broken down easily and good contact is directly established with the highly conductive copper metal.
On the other hand, the stubborn aluminium oxide is harder to dislodge, reappears quickly and develops a high electrical contact resistance leading to a loss of contact. The aluminium oxide layer also prevents non-mechanical connections such as soldering, which is only possible after applying a layer of tin, copper, or nickel.
When dissimilar metals come in contact, galvanic action occurs – material is transferred from the more positive metal or anode to the more negative metal or cathode. The intensity of the action depends on the respective positions of the metals in the periodic table.
Aluminium is the most anodic of engineering metals and copper the most cathodic. Therefore, at wire terminations with connectors which are invariably made of copper or brass (a copper alloy), aluminium loses material through galvanic action, leading to a loss of contact. The presence of water condensation which is to be expected in outdoor distribution transformers worsens this galvanic action. In contrast, copper wires, being the same element as the connectors, have no galvanic action.
Elongation and flow
Aluminium flows away from the termination under pressure. Furthermore, the linear coefficient of thermal expansion of aluminium is about 35% higher than that of copper. This plays a significant role in rigid connections, in which the change in dimensions by heat in combination with the flow may result in loss of contact at the termination. The only way to avoid these problems is to check all terminals periodically, and tighten screws and connecting devices.
Copper being harder, stronger and more ductile, expands less and does not flow at terminations and consequently does not require periodic inspection and tightening of screws.
In all instances, the loss of contact of aluminium wires at terminations leads to local overheating and eventually to sparking and fires.
Elektroinstallation de 22/2008: “Aluminium und Kupfer richtig verbinden”, “Sechskant-Pressung für Aluminium und Kupfer”