Pure aluminium and its alloys, mainly containing manganese (Mn), magnesium (Mg) as well as magnesium and silicon (MgSi), have consistently shown excellent corrosion resistance. Alloys using copper and zinc as alloying elements are less corrosion resistant.
In general, it may be said that there is only a slight difference in corrosion resistance between all common commercial aluminium alloys.
The explanation for aluminium’s good corrosion resistance lies in the natural oxide layer that is formed spontaneously, creating a seal and insulating layer. Moreover, the layer is stable (insoluble) within a broad pH interval, the atmospheric pH range of pH 4 to pH 9. This layer also has a slight propensity to absorb SO2.
In unfavourable conditions, aluminium may be subject to galvanic corrosion due to its electronegative nature, i.e. it is low down in the contact series for various metals. Aluminium is non-precious compared to steel, copper, nickel and passive in relation to stainless steel. Only magnesium and zinc are more non-precious and can serve as a sacrificial anode or other cathode protection for aluminium.
Galvanic corrosion occurs when aluminium comes into contact with another more precious metal or metal ions in solution. Contact must occur through direct metallic contact or via an electrolyte.
The effective method of preventing galvanic corrosion should be considered as early on as the construction phase:
- Use a uniform material
- Avoid using steel and copper alloys in the vicinity of aluminium
- Insulate against surrounding foreign metallic materials
- Use attachments of acid-proof stainless steel or aluminium
- Use anodised aluminium where possible
- Avoid standing water or areas of moisture
- Make contingencies for runoff, drainage and ventilation
- Avoid contact with strong alkalines.