The effects of the chemical reaction between water and aluminium

Aluminium power cables can potentially fail due to the following reasons: corrosion, thermal expansion and the reaction of water with aluminium. This article looks at the latter.

What happens when water meets aluminium?

Water can gain entrance into a cable during shipping, handling, outside storage, accidental damage or cable joint or termination failures. Due to its highly negative redox potential, aluminium reacts with water to produce hydrogen gas according to the equation: 2Al + 3H2O → 3H2 + Al2O3.This chemical reaction may be of particular importance when it occurs between the strands of an aluminium conductor. There is evidence that the result of such a reaction may be a significant build-up of hydrogen gas.

What affects the build-up of hydrogen?

The magnitude of the potential build-up of hydrogen is impossible to predict. Factors which could influence it include:

• Total volume of air space available within the conductor.

• Volume of water contained within the conductor.

• Conductor temperature.

• Available pure aluminium which could contact the water. The aluminium oxide layer must be penetrated, either chemically or mechanically to expose pure aluminium for the reaction to occur.

Is it a problem?

The formation of hydrogen can often lead to high pressures inside the cable, with the potential for a damaging expansion of the cable insulation structure. For example, if the pressure of hydrogen continues to increase, it may become sufficient to cause deformation at lesser constrained points in the system, such as joints or terminations, resulting in partial discharge activities, tracking and subsequent failure or even destruction of the cable. This failure mechanism may not always be recognized as being caused by hydrogen due to the lack of effective failure investigation procedures for particular distribution cables.

What’s the solution?

The most effective solution is to prevent water from entering the power conductor system in the first place, by applying radial and longitudinal water blocking protection methods. However, these protection methods are not always present.

A second and just as effective solution is to use copper instead of aluminium. As copper does not react with water, the problem is solved.

Reference

G.J.Luzzi, Gas pressure build-up in aluminium conductor cable, IEEE Transactions on Power Apparatus and Systems, Vol, PAS-101, No. 6, June 1982.