Corrosion of aluminium and copper in cable conductors

Corrosion – typically defined as the deterioration of metals through the combined actions of oxygen, water, other metals and salts – is a well-known degradation phenomenon which under some circumstances can be ”life threatening”.

Corrosion through exposure to oxygen

Aluminium oxidizes readily when exposed to air. A strongly attached, hard outer layer of electrically insulating oxide quickly forms around the metal [1]. Copper also oxidizes when exposed to air, but to a much lesser extent. The oxide which forms is relatively soft and – contrary to aluminium – is conductive, although not as conductive as the base metal.

Galvanic corrosion

Galvanic corrosion [2] can occur when dissimilar metals are in contact with each other and an electrolyte. For aluminium, a reactive metal in the galvanic series, this is the most common cause of corrosion. When aluminium comes into contact with a more cathodic material it acts as a sacrificial anode and becomes susceptible to corrosion. Copper, which is a relatively noble metal, does not usually exhibit a great deal of galvanic corrosion.

Consequences of corrosion

Corrosion can become threatening for two basic reasons:

  • Loss of material, and consequently loss of vital functions of the aluminium conductor and aluminium laminated covering, inevitably resulting in failure.
  • Introduction of an additional resistance, resulting in heat development and ultimately in failure. This is particularly important when considering the installation of a connector.

Corrosion: aluminium vs. copper

The corrosion of aluminium is generally recognized to be a major problem, although work is still being done to fully understand the mechanism, its impact on reliability [3] and the development of related protection methods. However, particularly when preparing joints on aluminium conductors, attention must be paid to the connector in regard to the phenomenon of oxidation. The oxide layer should be removed, and often an oxide-inhibiting compound can be applied to reduce oxidation.

For copper, corrosion is not an issue. Copper is resistant to most organic chemicals and can operate indefinitely in most industrial environments. A green patina may be formed after long exposure to the atmosphere, but this is a function of the protective surface film and does not indicate a harmful attack. The protection of copper is in fact unnecessary, even when used in offshore installations when it is exposed to a salty atmosphere.

References

1. R. Frank, C. Morton: Comparative corrosion and current burst testing of copper and aluminum electrical power connectors, IEEE Industry Applications Conference 2005.

2. A. Mak: Corrosion of steel, aluminum and copper in electrical applications, General Cable publication.

3. S. Pelissou, J. Cote, R. Savage, S. St-Antoine: Influence of corroded conductors on the performance of MV extruded cables, Jicable 03.

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