Galvanic corrosion (Topic 11004)

When two metallic materials are in contact in the presence of a corrosive electrolyte, corrosion will increase on the least noble material (the anode) and decrease on the noblest (the cathode). The increase in corrosion is called galvanic corrosion.

Since aluminium is less noble than most commonly used construction materials, with the exception of zinc, cadmium and magnesium, this can be a serious corrosion form with aluminium. The protective effect from the oxide layer can be seriously deteriorated by the coupling to a nobler material, this is especially dangerous in atmospheres or waters with high concentrations of chlorides or other aggressive species.

The main factors influencing the galvanic corrosion rate.

There is relatively small difference in the galvanic corrosion rate for different aluminium alloys, because the corrosion rate is largely decided by the cathodic properties of the noblest material. A galvanic connection will very often amplify exfoliation, intergranular and stress corrosion in alloys susceptible to these corrosion forms.

There are ways to avoid or minimize galvanic corrosion.

It is possible to utilize the galvanic corrosion in a positive manner. The most common is to use aluminium for cathodic protection as sacrificial anodes or as a surface coating. But it is also possible to connect aluminium parts to critical steel parts, the aluminium will act as a sacrificial anode and protect the steel, but the aluminium and steel will have to be in the same electrolyte or no effect will be seen. Aluminium alloys used for sacrificial anodes are alloyed with elements which inhibit the formation of a protective oxide layer and thus prevent passivation.